national health and medical research council levels of evidence

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

What is the “Best Evidence”?

What is “the best available evidence”? The hierarchy of evidence is a core principal of Evidence-Based Practice (EBP) and attempts to address this question. The evidence hierarchy allows you to take a top-down approach to locating the best evidence whereby you first search for a recent well-conducted systematic review and if that is not available, then move down to the next level of evidence to answer your question.

EBP hierarchies rank study types based on the rigor (strength and precision) of their research methods. Different hierarchies exist for different question types, and even experts may disagree on the exact rank of information in the evidence hierarchies. The following image represents the hierarchy of evidence provided by the National Health and Medical Research Council (NHMRC). 1

Most experts agree that the higher up the hierarchy the study design is positioned, the more rigorous the methodology and hence the more likely it is that the study design can minimize the effect of bias on the results of the study. In most evidence hierarchies current, well designed systematic reviews and meta-analyses are at the top of the pyramid, and expert opinion and anecdotal experience are at the bottom. 2

Systematic Reviews versus Primary Studies: What’s Best?

Systematic reviews and meta analyses.

Well done systematic reviews, with or without an included meta-analysis, are generally considered to provide the best evidence for all question types as they are based on the findings of multiple studies that were identified in comprehensive, systematic literature searches. However, the position of systematic reviews at the top of the evidence hierarchy is not an absolute. For example:

• The process of a rigorous systematic review can take years to complete and findings can therefore be superseded by more recent evidence.
• The methodological rigor and strength of findings must be appraised by the reader before being applied to patients.
• A large, well conducted Randomized Controlled Trial (RCT) may provide more convincing evidence than a systematic review of smaller RCTs. 4

Primary Studies

If a current, well designed systematic review is not available, go to primary studies to answer your question. The best research designs for a primary study varies depending on the question type. The table below lists optimal study methodologies for the main types of questions.

Note that the  Clinical Queries   filter available in some databases such as PubMed and CINAHL matches the question type to studies with appropriate research designs.

When searching primary literature, look first for reports of clinical trials that used the best research designs. Remember as you search, though, that the best available evidence may not come from the optimal study type. For example, if treatment effects found in well designed cohort studies are sufficiently large and consistent, those cohort studies may provide more convincing evidence than the findings of a weaker RCT.

Systematic Reviews and Narrative Reviews: What’s the Difference?

What is a systematic review.

A systematic review synthesizes the results from all available studies in a particular area, and provides a thorough analysis of the results, strengths and weaknesses of the collated studies.  A systematic review has several qualities:

• It addresses a focused, clearly formulated question.
• It uses systematic and explicit methods:

a. to identify, select and critically appraise relevant research, and b. to collect and analyze data from the studies that are included in the review

Systematic reviews may or may not include a meta-analysis used to summaries and analyze the statistical results of included studies. This requires the studies to have the same outcome measure.

What is a Narrative Review?

Narrative reviews (often just called Reviews) are opinion with selective illustrations from the literature. They do not qualify as adequate evidence to answer clinical questions. Rather than answering a specific clinical question, they provide an overview of the research landscape on a given topic and so maybe useful for background information. Narrative reviews usually lack systematic search protocols or explicit criteria for selecting and appraising evidence and are therefore very prone to bias. 5

Filtered versus Unfiltered Information

Filtered information appraises the quality of a study and recommend its application in practice. The critical appraisal of the individual articles has already been done for you—which is a great time saver. Because the critical appraisal has been completed, filtered literature is appropriate to use for clinical decision-making at the point-of-care. In addition to saving time, filtered literature will often provide a more definitive answer than individual research reports. Examples of filtered resources include, Cochrane Database of Systematic Reviews ,  BMJ Clinical Evidence , and  ACP Journal Club .

Unfiltered information  are original research studies that have not yet been synthesized or aggregated. As such, they are the more difficult to read, interpret, and apply to practice.  Examples of unfiltered resources include,  CINAHL ,  EMBASE ,  Medline , and  PubMe d . 3

The Cochrane Library

Cochrane Library

The  Cochrane Collaboration  is an international voluntary organization that prepares, maintains and promotes the accessibility of systematic reviews of the effects of healthcare.

The Cochrane Library is a database from the Cochrane Collaboration that allows simultaneous searching of six EBP databases. Cochrane Reviews  are systematic reviews authored by members of the Cochrane Collaboration and available via  The Cochrane Database of Systematic Reviews . They are widely recognized as the gold standard in systematic reviews due to the rigorous methodology used.

Abstracts of completed Cochrane Reviews are freely available through PubMed and Meta-Search engines such as TRIP database.

National access to the Cochrane Library  is provided by the Australian Government via the National Health and Medical Research Council (NHMRC).

1. National Health and Medical Research Council. (2009).  NHMRC Levels of Evidence and Grades for Recommendations for Developers of Clinical Practice Guidelines . Retrieved 2 July, 2014 from:  https://www.nhmrc.gov.au/_files_nhmrc/file/guidelines/developers/nhmrc_levels_grades_evidence_120423.pdf

2. Hoffman, T., Bennett, S., & Del Mar, C. (2013).  Evidence-Based Practice: Across the Health Professions   (2nd ed.). Chatswood, NSW: Elsevier.

3. Kendall, S. (2008). Evidence-based resources simplified.  Canadian Family Physician , 54, 241-243

4. Davidson, M., & Iles, R. (2013). Evidence-based practice in therapeutic health care. In, Liamputtong, P. (ed.).  Research Methods in Health: Foundations for Evidence-Based Practice  (2nd ed.). South Melbourne: Oxford University Press.

5. Cook, D., Mulrow, C., & Haynes, R. (1997). Systematic reviews: synthesis of best evidence for clinical decisions.  Annals of Internal Medicine , 126, 376–80.

Applying Research in Practice Copyright © by Duy Nguyen is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

Share This Book

Evidence-Based Practice in Health

• Introduction
• PICO Framework and the Question Statement
• Types of Clinical Question
• Hierarchy of Evidence

The Evidence Hierarchy: What is the "Best Evidence"?

Systematic reviews versus primary studies: what's best, systematic reviews and narrative reviews: what's the difference, filtered versus unfiltered information, the cochrane library.

• Selecting a Resource
• Searching PubMed
• Module 3: Appraise
• Module 4: Apply
• Module 5: Audit
• Reference Shelf

What is "the best available evidence"?  The hierarchy of evidence is a core principal of Evidence-Based Practice (EBP) and attempts to address this question.  The evidence higherarchy allows you to take a top-down approach to locating the best evidence whereby you first search for a recent well-conducted systematic review and if that is not available, then move down to the next level of evidence to answer your question.

EBP hierarchies rank study types based on the rigour (strength and precision) of their research methods.  Different hierarchies exist for different question types, and even experts may disagree on the exact rank of information in the evidence hierarchies.  The following image represents the hierarchy of evidence provided by the National Health and Medical Research Council (NHMRC). 1

Most experts agree that the higher up the hierarchy the study design is positioned, the more rigorous the methodology and hence the more likely it is that the study design can minimise the effect of bias on the results of the study.  In most evidence hierachies current, well designed systematic reviews and meta-analyses are at the top of the pyramid, and expert opinion and anecdotal experience are at the bottom. 2

Systematic Reviews and Meta Analyses

Well done systematic reviews, with or without an included meta-analysis, are generally considered to provide the best evidence for all question types as they are based on the findings of multiple studies that were identified in comprehensive, systematic literature searches.  However, the position of systematic reviews at the top of the evidence hierarchy is not an absolute.  For example:

• The process of a rigorous systematic review can take years to complete and findings can therefore be superseded by more recent evidence.
• The methodological rigor and strength of findings must be appraised by the reader before being applied to patients.
• A large, well conducted Randomised Controlled Trial (RCT) may provide more convincing evidence than a systematic review of smaller RCTs. 4

Primary Studies

If a current, well designed systematic review is not available, go to primary studies to answer your question. The best research designs for a primary study varies depending on the question type.  The table below lists optimal study methodologies for the main types of questions.

Note that the Clinical Queries filter available in some databases such as PubMed and CINAHL matches the question type to studies with appropriate research designs. When searching primary literature, look first for reports of clinical trials that used the best research designs. Remember as you search, though, that the best available evidence may not come from the optimal study type. For example, if treatment effects found in well designed cohort studies are sufficiently large and consistent, those cohort studies may provide more convincing evidence than the findings of a weaker RCT.

What is a Systematic Review?

A systematic review synthesises the results from all available studies in a particular area, and provides a thorough analysis of the results, strengths and weaknesses of the collated studies.  A systematic review has several qualities:

• It addresses a focused, clearly formulated question.
• It uses systematic and explicit methods:

                  a. to identify, select and critically appraise relevant research, and                   b. to collect and analyse data from the studies that are included in the review

Systematic reviews may or may not include a meta-analysis used to summarise and analyse the statistical results of included studies. This requires the studies to have the same outcome measure.

What is a Narrative Review?

Narrative reviews (often just called Reviews) are opinion with selective illustrations from the literature.  They do not qualify as adequate evidence to answer clinical questions.  Rather than answering a specific clinical question, they provide an overview of the research landscape on a given topic and so maybe useful for background information.  Narrative reviews usually lack systematic search protocols or explicit criteria for selecting and appraising evidence and are threfore very prone to bias. 5

Filtered information appraises the quality of a study and recommend its application in practice.  The critical appraisal of the individual articles has already been done for you—which is a great time saver.  Because the critical appraisal has been completed, filtered literature is appropriate to use for clinical decision-making at the point-of-care. In addition to saving time, filtered literature will often provide a more definitive answer than individual research reports.  Examples of filtered resources include, Cochrane Database of Systematic Reviews , BMJ Clincial Evidence , and ACP Journal Club .

Unfiltered information are original research studies that have not yet been synthesized or aggregated. As such, they are the more difficult to read, interpret, and apply to practice.  Examples of unfiltered resources include, CINAHL , EMBASE , Medline , and PubMe d . 3

The Cochrane Collaboration is an international voluntary organization that prepares, maintains and promotes the accessibility of systematic reviews of the effects of healthcare. 

The Cochrane Library is a database from the Cochrane Collaboration that allows simultaneous searching of six EBP databases.  Cochrane Reviews are systematic reviews authored by members of the Cochrane Collaboration and available via The Cochrane Database of Systematic Reviews .  They are widely recognised as the gold standard in systematic reviews due to the rigorous methodology used. 

Abstracts of completed Cochrane Reviews are freely available through PubMed and Meta-Search engines such as TRIP database. 

National access to the Cochrane Library is provided by the Australian Government via the National Health and Medical Research Council (NHMRC).

1. National Health and Medical Research Council. (2009). [Hierarchy of Evidence] . Retrieved 2 July, 2014 from: https://www.nhmrc.gov.au/

2. Hoffman, T., Bennett, S., & Del Mar, C. (2013). Evidence-Based Practice: Across the Health Professions (2nd ed.). Chatswood, NSW: Elsevier.

3. Kendall, S. (2008). Evidence-based resources simplified. Canadian Family Physician , 54, 241-243

4. Davidson, M., & Iles, R. (2013). Evidence-based practice in therapeutic health care. In, Liamputtong, P. (ed.). Research Methods in Health: Foundations for Evidence-Based Practice (2nd ed.). South Melbourne: Oxford University Press.

5. Cook, D., Mulrow, C., & Haynes, R. (1997). Systematic reviews: synthesis of best evidence for clinical decisions. Annals of Internal Medicine , 126, 376–80.

• << Previous: Types of Clinical Question
• Next: Module 2: Acquire >>
• Last Updated: Jul 24, 2023 4:08 PM
• URL: https://canberra.libguides.com/evidence

Levels and Grades of Evidence

Chris nickson.

• Nov 3, 2020

Reviewed and revised 26 August 2015

• different systems of categorising the quality of evidence, and individual studies, have been developed
• primarily used in evidence-based clinical guidelines

NHMRC LEVELS OF EVIDENCE

The following is the designation used by the Australian National Health and Medical Research Council (NHMRC):

• Evidence obtained from a systematic review of all relevant randomised controlled trials.
• Evidence obtained from at least one properly designed randomised controlled trial.
• Evidence obtained from well-designed pseudo-randomised controlled trials (alternate allocation or some other method).
• Evidence obtained from comparative studies with concurrent controls and allocation not randomised (cohort studies), case control studies, or interrupted time series with a control group.
• Evidence obtained from comparative studies with historical control, two or more single-arm studies, or interrupted time series without a parallel control group.
• Evidence obtained from case series, either post-test or pre-test and post-test.

OXFORD CENTRE FOR EVIDENCE BASED MEDICINE 2011

• I – systemic review of all relevant RCTs OR an n=1 RCT
• II – Randomized trial or observational study with dramatic effect
• III – Non-randomized controlled cohort/follow-up study (observational)
• IV – Case-series, case-control studies, or historically controlled studies
• V – mechanism-based reasong (expert opinion, based on physiology, animal or laboratory studies)
• A – consistent level 1 studies
• B – consistent level 2 or 3 studies or extrapolations from level 1 studies
• C – level 4 studies or extrapolations from level 2 or 3 studies
• D – level 5 evidence or troubling inconsistent or inconclusive studies of any level

References and Links

• CCC — Types of Research Studies

FOAM and web resources

• Oxford Centre for Evidence-based Medicine – OCEMB Levels of Evidence System
• www.GradeWorkingGroup.org

Critical Care

Chris is an Intensivist and ECMO specialist at the  Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University . He is a co-founder of the  Australia and New Zealand Clinician Educator Network  (ANZCEN) and is the Lead for the  ANZCEN Clinician Educator Incubator  programme. He is on the Board of Directors for the  Intensive Care Foundation  and is a First Part Examiner for the  College of Intensive Care Medicine . He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website,  INTENSIVE .  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the  FOAM  movement (Free Open-Access Medical education) and is co-creator of  litfl.com , the  RAGE podcast , the  Resuscitology  course, and the  SMACC  conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is  @precordialthump .

| INTENSIVE | RAGE | Resuscitology | SMACC

Leave a Reply Cancel reply

This site uses Akismet to reduce spam. Learn how your comment data is processed .

Systematic Reviews

• Levels of Evidence
• Evidence Pyramid
• Joanna Briggs Institute

The evidence pyramid is often used to illustrate the development of evidence. At the base of the pyramid is animal research and laboratory studies – this is where ideas are first developed. As you progress up the pyramid the amount of information available decreases in volume, but increases in relevance to the clinical setting.

Meta Analysis  – systematic review that uses quantitative methods to synthesize and summarize the results.

Systematic Review  – summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies and that uses appropriate st atistical techniques to combine these valid studies.

Randomized Controlled Trial – Participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest.

Cohort Study – Involves identification of two groups (cohorts) of patients, one which received the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.

Case Control Study – study which involves identifying patients who have the outcome of interest (cases) and patients without the same outcome (controls), and looking back to see if they had the exposure of interest.

Case Series   – report on a series of patients with an outcome of interest. No control group is involved.

• Levels of Evidence from The Centre for Evidence-Based Medicine
• The JBI Model of Evidence Based Healthcare
• How to Use the Evidence: Assessment and Application of Scientific Evidence From the National Health and Medical Research Council (NHMRC) of Australia. Book must be downloaded; not available to read online.

When searching for evidence to answer clinical questions, aim to identify the highest level of available evidence. Evidence hierarchies can help you strategically identify which resources to use for finding evidence, as well as which search results are most likely to be "best".                                             

Image source: Evidence-Based Practice: Study Design from Duke University Medical Center Library & Archives. This work is licensed under a Creativ e Commons Attribution-ShareAlike 4.0 International License .

The hierarchy of evidence (also known as the evidence-based pyramid) is depicted as a triangular representation of the levels of evidence with the strongest evidence at the top which progresses down through evidence with decreasing strength. At the top of the pyramid are research syntheses, such as Meta-Analyses and Systematic Reviews, the strongest forms of evidence. Below research syntheses are primary research studies progressing from experimental studies, such as Randomized Controlled Trials, to observational studies, such as Cohort Studies, Case-Control Studies, Cross-Sectional Studies, Case Series, and Case Reports. Non-Human Animal Studies and Laboratory Studies occupy the lowest level of evidence at the base of the pyramid.

• << Previous: What is a Systematic Review?
• Next: Locating Systematic Reviews >>
• Getting Started
• What is a Systematic Review?
• Locating Systematic Reviews
• Searching Systematically
• Developing Answerable Questions
• Identifying Synonyms & Related Terms
• Using Truncation and Wildcards
• Identifying Search Limits/Exclusion Criteria
• Keyword vs. Subject Searching
• Where to Search
• Search Filters
• Sensitivity vs. Precision
• Core Databases
• Other Databases
• Clinical Trial Registries
• Conference Presentations
• Databases Indexing Grey Literature
• Web Searching
• Handsearching
• Citation Indexes
• Documenting the Search Process
• Managing your Review

Research Support

• Last Updated: Jun 6, 2024 9:14 AM
• URL: https://guides.library.ucdavis.edu/systematic-reviews

Levels of Evidence

Levels of evidence (or hierarchy of evidence) is a system used to rank medical studies based on the quality and reliability of their designs. The levels of evidence are commonly depicted in a pyramid model that illustrates both the quality and quantity of available evidence. The higher the position on the pyramid, the stronger the evidence. 1 Each level builds on data and research previously developed in the lower tiers.

Levels of evidence pyramids are often divided into two or three sections. The top section consists of filtered (secondary) evidence, including systematic reviews, meta-analyses, and critical appraisals. The section below includes unfiltered (primary) evidence, including randomized controlled trials, cohort studies, case-controlled studies, case series, and case reports. 1 Some models include an additional bottom segment for background information and expert opinion. 2

Definitions

Systematic Review and Meta-Analysis

A systematic review synthesizes the results from available studies of a particular health topic, answering a specific research question by collecting and evaluating all research evidence that fits the reviewer’s selection criteria. 3 The most well-known collection of systematic reviews is the Cochrane Database of Systematic Reviews .

Systematic reviews can include meta-analyses in which statistical methods are applied to evaluate and synthesize quantitative results from multiple studies.

A randomized controlled trial is a prospective study that measures the efficacy of an intervention or treatment. Subjects are randomly assigned to either an experimental group or a control group; the control group receives a placebo or sham intervention, while the experimental group receives the intervention being studied. Randomizing subjects is effective at removing bias, thus increasing the validity of the research. RCTs are frequently blinded so that neither the subjects (single blind), nor the clinicians (double blind), nor the researchers (triple blind) know in which group the subjects are placed. 4

A cohort study is a type of observational study, meaning that no intervention is taken among the subjects. It is also a type of longitudinal study in which research subjects are followed over a period of time. 5 A cohort study can be either prospective, which collects new data over time, or retrospective, which uses previously acquired data or medical records. This type of study examines a group of people who share a common trait or exposure and are assessed based on whether they develop an outcome of interest. An example of a prospective cohort study is a study that determines which subjects smoke and then many years later assesses the incidence of lung cancer in both smokers and non-smokers.

A case-control study is another type of observational study. It is also a type of retrospective study that looks back in time to assess information. A case-control study compares people who have the specified condition or outcome being studied (known as “cases”) with people who do not have the condition or outcome (known as “controls”). 6 An example of a case-control study is a study that assesses the lifetime smoking exposure of patients with and without lung cancer.

A case report is a detailed report of the presentation, diagnosis, treatment, treatment response, and follow-up after treatment of an individual patient. A case series is a group of case reports involving patients who share similar characteristics. A case series is observational and can be conducted either retrospectively or prospectively.

Also called a prevalence study, a cross-sectional study examines subjects at a single point in time. By definition, a cross-sectional study is only observational. 7 An example of a cross-sectional study is a survey of a population to determine the prevalence of lung cancer.

Filtered vs. Unfiltered Information

Filtered (secondary) levels of evidence include information that has been previously collected, analyzed, and aggregated by expert analysis and review. Filtered levels of evidence are placed above unfiltered levels of evidence on the pyramid. Examples of filtered levels of evidence are systematic reviews and meta-analyses.

Unfiltered (primary) evidence includes original research studies, including randomized controlled trials and case-control studies. They are often published in peer-reviewed journals. 8 However, these studies have not been subjected to additional analysis and review beyond that of the peer reviewers for each study. In most cases, unfiltered levels of evidence are difficult to apply in clinical decision-making. 9

In 1972, Archibald Cochrane, a physician from Scotland, wrote Effectiveness and Efficiency, in which he argued that decisions about medical treatment should be based on a systematic review of the available clinical evidence. Cochrane proposed an international collaboration of researchers to systematically review the best clinical studies in each specialty. 10

In 1979, the Canadian Task Force on the Periodic Health Examination published a ranking system for medical evidence, proposing four quality levels: 11,12

• I: Evidence obtained from at least one properly designed randomized controlled trial
• II-1: Evidence obtained from a well-designed cohort or case-control analytic study, preferably from more than one center or research group
• II-2: Evidence obtained from comparisons between times or places with or without the intervention
• III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees

The U.S. Preventive Services Task Force (USPSTF) adopted a modified version of the Canadian Task Force’s categorization in 1988: 13,14

• II-1: Evidence obtained from well-designed controlled trials without randomization
• II-2: Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one center or research group
• II-3: Evidence obtained from multiple time series designs with or without the intervention; dramatic results in uncontrolled trials might also be regarded as this type of evidence

The physician Gordon Guyatt, who in 1991 coined the term “evidence-based medicine,” proposed another approach to classifying the strength of recommendations in Users' Guides to the Medical Literature . 15, 16 Referencing Guyatt’s paper, Trisha Greenhalgh summarized his revised hierarchy as follows: 17

• Systematic reviews and meta-analyses
• Randomized controlled trials with definitive results (confidence intervals that do not overlap the threshold of a clinically significant effect)
• Randomized controlled trials with non-definitive results (a point estimate that suggests a clinically significant effect but with confidence intervals overlapping the threshold for this effect)
• Cohort studies
• Case-control studies
• Cross-sectional surveys
• Case reports

Evidence levels can vary based on the clinical question being asked (i.e., the categorization of evidence for a medical treatment may differ from evidence for determining disease prevalence). For example, The Centre for Evidence-Based Medicine and American Society of Plastic Surgeons published tables specific to therapeutic, diagnostic, and prognostic studies. 18,19

• Murad MH, Asi N, Alsawas M, Alahdab F. New evidence pyramid. BMJ Evidence Based Medicine. 2016;21(4):125–127.
• Illustration adapted from model displayed in “Evidence-Based Practice in Health”. The model is attributed to the National Health and Medical Research Council. NHMRC levels of evidence and grades for recommendations for developers of guidelines. Retrieved from University of Canberra Library.
• Turner M. “Evidence-Based Practice in Health”. 2014. Retrieved from University of Canberra website.
• Hariton E, Locascio JJ. Randomised controlled trials—The gold standard for effectiveness research: Study design: Randomised controlled trials. BJOG. 2018;125(13):1716.
• Barrett D, Noble H. What are cohort studies? Evid Based Nur. 2019;22(4):95–6.
• Himmelfarb Health Sciences Library. Study design 101: Case control study. 2019.
• Singh Setia M. Methodology Series Module 3: Cross-sectional Studies. Indian J Dermatol. 2016;61(3):261–264.
• Northern Virginia Community College. Evidence-based practice for health professionals. 2022.
• Kendall S. Evidence-based resources simplified. Can Fam Physician. 2008;54(2):241–243.
• Stavrou A, Challoumas D, Dimitrakakis G. Archibald Cochrane (1909–1988): The father of evidence-based medicine. Interact Cardiovasc Thorac Surg. 2014;18(1):121–124.
• Spitzer WO, et al. The periodic health examination. Canadian Task Force on the Periodic Health Examination. Can Med Assoc J. 1979;121(9):1193–1254.
• Burns PB, Rohrich RJ, Chung KC. The Levels of Evidence and their Role in Evidence-Based Medicine. Plastic and Reconstructive Surgery. 2010:128(1):305–310.
• U.S. Preventive Services Task Force. (as of 2018). Grade definitions.
• U.S. Preventive Services Task Force. Guide to Clinical Preventive Services: Report of the U.S. Preventive Services Task Force. DIANE Publishing, 1989. ISBN 1568062974.
• Guyatt GH, Sackett DL, Sinclair JC, Hayward R, Cook DJ, Cook RJ. Users’ guides to the medical literature IX. A method for grading health care recommendations. Evidence-Based Medicine Working Group. JAMA. 1995;274(22):1800–1804.
• Zimerman AL. Evidence-Based Medicine: A Short History of a Modern Medical Movement. Virtual Mentor. 2013;15(1):71–76.
• Greenhalgh T. How to read a paper. Getting your bearings (deciding what the paper is about). BMJ. 1997;315(7102):243–246. doi:10.1136/bmj.315.7102.243
• Sullivan D, Chung KC, Eaves FF 3rd, Rohrich RJ. The level of evidence pyramid: Indicating levels of evidence in Plastic and Reconstructive Surgery articles. Plast Reconstr Surg. 2011;128(1):311–314. doi:10.1097/PRS.0b013e3182195826
• Oxford Centre for Evidence-Based Medicine: Levels of evidence. March 2009. CEBM.

Contributors

• Moira Tannenbaum, MSN
• Stacy Sebastian, MD
• Brian Sullivan, MD

Published: August 17, 2021

Updated: November 1, 2022

Return to EBM

Levels of Evidence

Not all evidence is the same.  Clearly, results from a systematic review of well conducted double-blind randomised controlled trials are much more reliable than anecdotal opinion.

NHMRC Levels of Evidence

The following is the designation used by the Australian National Health and Medical Research Council (NHMRC) [1] :

Level I Evidence obtained from a systematic review of all relevant randomised controlled trials. Level II Evidence obtained from at least one properly designed randomised controlled trial. Level III-1 Evidence obtained from well-designed pseudo-randomised controlled trials (alternate allocation or some other method). Level III-2 Evidence obtained from comparative studies with concurrent controls and allocation not randomised ( cohort studies ), case control studies , or interrupted time series with a control group. Level III-3 Evidence obtained from comparative studies with historical control, two or more single-arm studies, or interrupted time series without a parallel control group. Level IV Evidence obtained from case series , either post-test or pre-test and post-test.

Oxford Centre for Evidence Based Medicine

This is the system used by the UK National Health Service (NHS).  The following has been divided into a simplified version of the “grade of recommendation” system first (the oft seen grades A to D) and then the more detailed levels of evidence [2] .

• Consistent Randomised Controlled Clinical Trial, cohort study, all or none, clinical decision rule validated in different populations.
• Consistent Retrospective Cohort, Exploratory Cohort, Ecological Study, Outcomes Research, case-control study; or extrapolations from level A studies.
• Case-series study or extrapolations from level B studies.
• Expert opinion without explicit critical appraisal, or based on physiology, bench research or first principles.

Levels of Evidence for Therapy/Prevention, Aetiology/Harm

Level 1a Systematic review with homogeneity (*) of randomised control trials Level 1b Individual randomised control trial with narrow confidence interval (studies with wide confidence interval should be tagged with a “-” at the end of their designated level). Level 1c All or none (met when all patients died before Rx became available, but some now survive on it; or when some patients died before the Rx became available but none now die on it). Level 2a Systematic review with homogeneity(*) of cohort studies Level 2b Individual cohort studies ; Low quality randomised control trials (e.g., < 80% follow up) Level 2c “Outcomes” Research; Ecological studies Level 3a Systematic review with homogeneity(*) of case-control studies Level 3b Individual case-control studies Level 4 Case series ; Poor quality cohort studies (failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same, objective way in both exposed and non-exposed individuals and/or failed to identify or appropriately control known confounders and/or failed to carry out a sufficient long and completely follow up); Poor quality case control studies (failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same, objective way in both cases and controls and/or failed to identify or appropriately control known confounders) Level 5 Expert opinion without explicit critical appraisal , or based on physiology , bench research or “first principles” . (*) A systematic review free of worrisome variations in the directions and degrees of results between individual studies. Not all systematic reviews with statistically significant heterogeneity need be worrisome, and not all worrisome heterogeneity need be statistically significant. Studies displaying worrisome heterogeneity should be tagged with a “-” at the end of their designated level.

Grade of Recommendation

A: consistent level 1 studies B: consistent level 2 or 3 studies or extrapolations from level 1 studies C: level 4 studies or extrapolations from level 2 or 3 studies D: level 5 evidence or troubling inconsistent or inconclusive studies of any level

• A guide to the development, implementation and evaluation of clinical practice guidelines.   NHMRC , 1999.
• Levels of Evidence (March 2009).   Centre for Evidence Based Medicine .  Retrieved from www.cebm.net on 5 October 2010.

Share this:

Permanent link to this article: https://evidencebasedmedicine.com.au/?page_id=30

In this section

Recent posts.

• N95 respirators vs surgical masks to prevent transmission of respiratory tract infections to staff in primary care
• Palmitoylethanolamide (PEA) as treatment for knee osteoarthritis pain
• Duloxetine as treatment for knee osteoarthritis pain
• Isopropyl alcohol nasal inhalation for nausea in adults
• EMDR as treatment for post-traumatic stress disorder
• Oral cimetidine as the treatment of common warts

Recent Comments

• Professor Maximilian Gruber on N95 respirators vs surgical masks to prevent transmission of respiratory tract infections to staff in primary care
• John Deery on Duct tape to treat cutaneous warts
• Carl on Is glucosamine effective for osteoarthritis pain?
• Ragel on Zinc for the common cold
• Michael Tam on Cranberry juice fails for urinary tract infections

© 2024 Morsels of Evidence.

Made with by Graphene Themes .

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings

Preview improvements coming to the PMC website in October 2024. Learn More or Try it out now .

• Advanced Search
• Journal List
• Int J Sports Phys Ther
• v.7(5); 2012 Oct

LEVELS OF EVIDENCE IN MEDICINE

Peter mcnair.

1 Health and Rehabilitation Research Institute, Auckland University of Technology, Auckland, New Zealand

Levels of evidence allow clinicians to appreciate the quality of a particular research paper quickly. The levels are generally set out in a hierarchical order, which is based largely upon the experimental design. While there are ideal designs for studies examining the effects of interventions, risk factors for a clinical condition or diagnostic testing, in most instances researchers have had to make compromises and these subsequently decrease the quality of their work. This paper provides information concerning how those compromises relate to subsequent levels that are given to a piece of research. It also provides an understanding of issues related to evaluating papers, and suggest ways in which the reader might discern how relevant a paper might be to one's clinical practice.

INTRODUCTION

During the 1990s, the term evidence based medicine (EBM) became notably more apparent in research and clinical literature. As the name suggests, it referred to examining the research evidence for making clinical decisions, and as such it was more firmly grounded in the assessment of the science supporting clinical decision‐making, rather than a reliance on the experiences and subjective perceptions of so called authorities or experts. 1 For EBM to have credibility, there needed to be a systematic manner in which clinical research was assessed, and this demanded the development of levels of evidence to ultimately appreciate and assess the quality of research available in answering a particular clinical question. Initially, efforts on assessment of quality were focused upon intervention studies, examining the degree of effectiveness of treatments for clinical disorders, however, in recent years such efforts have expanded to include other key clinical research areas such as diagnosis and risk factors. The purpose of this paper is to describe the key elements that determine the levels of evidence that subsequently allow the most appropriate or efficacious clinical decision to be made for the patient.

STUDY DESIGN HIERARCHIES PROVIDE AN INITIAL STARTING POINT

Physical Therapists are often interested in studies that involve treatment interventions, identifying risk factors for succumbing to an injury or disease, and diagnosis of clinical conditions. In each of these areas, there are a number of different study designs that can be implemented. These designs may dictate the potential importance of the studies findings in its field. The design that a researcher chooses should be that which most appropriately answers the question being posed. 2 However in many cases, it reflects the resources that researchers have at their disposal and the practicalities of undertaking the research. Resources required for studies may involve physical space and equipment, expertise in data collection, administrative processing of data, statisticians for analyzing data, and patient availability. In most cases, a researcher does not have the opportunity to cover all of these resources to the maximum level possible. Because of this, compromises are made and these often affect the choice of design to be utilised during the research process.

In studies concerning interventions, risk factors and diagnosis, the strength of an experimental paper's design is rated upon a scale that has 4‐5 levels and may be regarded as a hierarchy with level 1 being the highest. In the current paper, the hierarchies presented are based on those recommended by the National Health and Medical Research Council of Australia. 3 However, there are others 4 and they generally follow the same pattern, being different only in the alphanumeric nomenclature given to the levels of the hierarchy (eg: 1a or IIa etc). While one design may be high in the hierarchy for a particular question to be answered, it may not fare so well for a different question. For instance, while a prospective cohort study may be very effective at identifying risk factors, such a design does not provide professionals with the best evidence of a treatment's effect on a particular clinical condition. For the latter, a randomised controlled trial (RCT) would be more appropriate. Thus, it is important to recognise that different study designs have particular features that may make them advantageous for answering a certain type of research question.

If possible, always look for systematic reviews when searching the literature. A Level 1 rating is reserved for a systematic review of the experimental papers. In such a paper, the quality of the designs and the findings of all the individual experimental papers are assessed in a systematic manner to provide an overall assessment or answer for a particular study question. However, it should be noted that not all systematic reviews automatically reach Level 1. If the papers that were reviewed were primarily of studies with poor designs, then the strength of evidence for the providing the answer to the question posed is lower, and the systematic review no matter how well it was conducted will not receive Level 1 status. 5 Thus, the experimental papers upon which the review is based should determine the validity and strength of the review's findings.

Even when a systematic review has utilised papers with the strongest possible designs, the professional needs to appreciate a number of other factors that will influence its importance. These include the number of papers that have been reported upon, and the consistency of the results across papers. One should also appreciate the degree to which the findings apply to the clinical population of interest and what the implications are in respect to applying them in clinical practice, that is, could they be reasonably implemented. On the above‐mentioned scale, the highest quality experimental designs are rated with a Level 2 and lesser‐rated designs receive Levels that decline to 4‐5.

Interventions

For studies examining treatment interventions, randomised controlled trials (RCTs) provide Level II evidence, the strongest level of evidence below a systematic review. Not surprisingly, the two key criteria for these study designs are the incorporation of at least one control group and the randomisation of participants. 6 Without a control group, it is impossible to determine how participants would have changed over time without the experimental intervention. For instance, changes may have occurred due to disease progression or spontaneous recovery. The specific conclusions that can be drawn regarding the experimental intervention are critically dependent on what the control group receives. For example, researchers could compare the effects of icing on acute knee pain to a control group who received no specific intervention, or they could give the control group a bag of peas that are at room temperature to place over their knee for the same period of time. In the first example, the only conclusion that could be drawn is that icing is more effective at reducing pain than no treatment, whereas in the latter example, by controlling for effects associated with receiving a physical intervention to the knee and for the time of application, a researcher could therefore make more specific conclusions regarding the effects of ice itself. In terms of randomisation, the crucial criterion for a RCT is that neither the participant nor the experimenter should be able to predict which group the participant will be allocated to. Commonly accepted randomisation procedures include a coin toss, random number generator, drawing group allocation from an envelope. While researchers may design more complex procedures to ensure that group characteristics are matched on important factors and that participant numbers are balanced between groups, the final determination of group allocation for each participant should be due to chance alone.

One step down from an RCT is a pseudo‐RCT, which provides Level III‐1 evidence. In these study designs, there is still an appropriate control group but group allocation is not strictly randomised. Group allocation in pseudo‐RCTs is dictated by a set rule such as date of birth or participant number. These are weaker randomisation procedures as the experimenter can have knowledge of the group to which a participant will be assigned. The ability to predict group allocation introduces bias into the study as this knowledge can affect the decision about whether to enter the participant into the trial, which may bias the results of the trial overall.

The next level of evidence, Level III‐2, incorporates non‐randomised controlled trials and two types of observational studies. Non‐randomised controlled trials have marked group selection bias. For example, participants may allocate themselves into groups by choosing to receive a treatment, or participants presenting to a particular treatment provider might be always allocated to the experimental intervention and those that present to another treatment provider might receive a control intervention only. Observational designs include cohorts in which a group of people who are exposed to a particular intervention are followed over time and their health outcomes compared to a similar group of people who were not exposed to the intervention. Another example of an observational study is the case‐control design, in which people with a selected condition are identified and their history of exposure to an intervention is compared to a similar group of people who do not have the condition. In all of these study designs, the researchers are not in control of group randomisation and thus the potential for selection bias is substantially higher than in RCTs. This selection bias means that there will be an inherent risk that confounding factors, or factors other than the intervention of interest, are influencing the results of the study. However, it is important to recognise that there are some research questions and interventions to which researchers cannot apply the principles of randomisation and have subjects assigned to different groups., e.g. abortion or obesity, or whether parachutes are an effective life saver. In such situations, the observational designs are the best or only alternative, and hence they can be extremely valuable. 7

The final group of studies providing Level III evidence (Level III‐3) are comparative studies with non‐controlled designs. These are non‐randomised studies where a group of people receiving the intervention of interest are compared with previous or historical information, or to another group receiving another intervention in another study. The key limitation of these studies is the lack of a concurrent control group, and thus it is not possible to determine the specific effects of the intervention in the population as there is not a suitable comparative group. The attempt to make up for the lack of a control group by comparing to historical data or other studies provides an improvement over non‐comparative studies (see case series below), but is still limited. For example, comparison to historical data on disease progression may be confounded by changes in disease management, specific characteristics of the participants tested, or variations in the assessment of outcome measures.

The lowest level of evidence (Level IV) is provided by case series that have no comparison group. These are usually pre‐test ‐ post‐test comparisons of outcomes following an intervention in a single group. Obviously, the lack of a control comparison severely limits the strength of the findings and the conclusions that could be drawn. These study designs will often incorporate the addition of a second pre‐test measure following a baseline, control period. This control period and additional baseline measure marginally strengthen the design of the study by enabling participants to serve “as their own control”. Case series study designs are commonly used for feasibility studies to demonstrate the potential efficacy, safety, or practicality of an intervention before implementation in a larger, more robust study. 8

Risk factors

In the intervention section above, we described observational study designs such as the prospective cohort and the case control. While not the best choice of design for examining interventions where subjects can be randomised into groups, they can be very powerful in the study of risk factors associated with the development of clinical conditions. 9 In the aetiology hierarchy, the strongest of the observational studies is the prospective cohort receiving level II. As the name suggests, it follows a group of similar individuals (eg: forestry workers) over time to examine whether a particular factor (eg: vibration from chain saw use) influences the occurrence of an outcome (osteoarthritis in the hand). A key point is that the occurrence of the outcome has not occurred at the commencement of the study. Such a design allows a consistent measurement of exposure across all the study participants and consistent measurement of the criteria that determines the outcome (eg: the presence of osteoarthritis in the hand). Cohort designs can be prospective or retrospective with the latter being at a lower hierarchal level. The key difference is that the data related to the exposure and the outcome has already been collected in the retrospective design. In many instances, the risk factor and/or outcome of interest was not the reason for the original study. 10 For example, while a prospective study may have primarily been run to examine vibration levels as a risk factor for osteoarthritis of the hand in forestry workers, data might also have been collected on specific safety procedures and injuries that occurred in this cohort. Such data can be linked retrospectively and associations between variables can provide important findings. However, because the retrospective study was not the original intention, the same degree of standardisation of the data collection procedures and the precision in which they were collected is unlikely to have been undertaken and therefore the design is not as strong as a prospective study.

At the next level in the hierarchy of designs for examining risk factors is the case‐control study. In this design two groups are identified, one that has a clinical condition of interest, and another that does not. For instance, a group of forestry workers with osteoarthritis of the hand would be the case group and they would be compared to a group of forestry workers without osteoarthritis of the hand. That comparison might involve examining potential physical risk factors, (e.g. tools used, tasks performed, times and volume of work) that were undertaken by both groups over a specified time to highlight a risk factor or set of factors that are different across the groups. This design is weaker than the cohort design as only the outcome (osteoarthritis of the hand) has the potential to have been measured in a standardised and precise manner. 10 Even then, one of the most notable criticisms of this design is that the criteria for being included in either the control or case groups may be insufficient to accurately represent those of the wider population with and without the condition of interest. 9 This is particularly so, when the case‐control design is targeting risk factors for a rare condition. Characterising risk factors associated with rare conditions is a key strength of the case control. The alternative, if one were to use a prospective cohort, means waiting for sufficient cases to contract a disease so that its risk factors might be characterised well, and that may never eventuate.

Cross sectional study designs and case series form the lowest level of the aetiology hierarchy. In the cross sectional design, data concerning each subject is often recorded at one point in time. For instance, a questionnaire might be sent to a district where forestry is a predominant industry. It might ask about the presence of osteoarthritis in the hand. In doing so, the prevalence of the disorder can be established. Some information related to exposure might also be collected and associations might be observed, but it is difficult to be confident in the validity of these associations. Thus, information gained from the cross‐sectional study is often a starting point that provides the impetus to use a more powerful design to substantiate the initial findings.

For diagnostic studies, the basic design utilized is very similar across most studies, and the higher levels of the hierarchy are based on meeting specific methodological criteria within that design. To receive Level II strength, the design is usually a prospective cohort, and the comparison it makes between a diagnostic test and a reference standard requires the following criteria: 11 All subjects should receive the reference standard, and that standard should be the best evidence available for determining whether the condition of interest is present. For studies, involving primary care, this will often be a scanning or electrophysiological procedure and might also include an anaesthetic block, while in studies involving tertiary care patients, the reference standard is often what is observed at surgery. The diagnostic test and the reference standard should also be completely independent of one another. It is crucial that the reference standard and the diagnostic tests are clearly described so that others can replicate them. The persons performing the diagnostic tests on the patients should not have knowledge of the results of the reference standard and similarly those performing the reference standard should have no knowledge of the results of the diagnostic test. The patients participating in the study must be well described, and represent those with mild as well as severe levels of the condition of interest who are recruited in a consecutive manner, and at the end of the study they are all accounted for.

Studies where the subjects are not consecutively recruited are assigned level III‐1 strength. When the criteria relating to reference standards are partially compromised, a study is regarded as level III‐2. When a study uses a group of subjects that don't include a wide spectrum of those likely to have the condition, or don't identify specific potential sub‐groupings that might affect the results, it is assigned level III‐3. Such studies are often case‐control designs where there are narrow criteria for inclusion in either the case or control groups, which can ultimately affect the generalizability of the results. 12 The lowest level (IV) is reserved for those studies that lack a reference standard.

IRRESPECTIVE OF DESIGN, THE QUALITY OF STUDIES IS IMPORTANT

While hierarchies provide the professional with a guide to how well a study design might answer a question, one must also consider how well that design has been implemented. 5 Within each design, there is a set of criteria that should be subscribed to, to make the design as robust as possible. The RCT may be at level II on the design hierarchy, and hence a good choice of design for studies examining the effects of an intervention. However, if that RCT has insufficient subject numbers to detect a reasonable difference across groups or blinding of subjects was not undertaken, or there were notable dropouts, then one should question the value of the results from that study, despite the design being the most appropriate. A study with a design lower on the hierarchy that has been undertaken well may provide more valid information.

There are numerous scales or checklists to choose from within the literature to assess the quality of individual research studies across the domains of interventions, aetiology, and diagnosis. The key sources of bias that might threaten the validity of the results of studies generally relates to the selection of patients, randomization, therapeutic regime, withdrawals, blinding, and statistical analyses. 6 Be aware that some checklists are extremely extensive 13 and include questions on issues that may not actually have the potential to bias the results, which is the primary reason for your assessment of the methodological quality.

The answers to checklist questions concerning methodological issues may be categorical (eg: bias present or not) or may be graded (e.g. 1 to 4). In some instances, the answers are weighted according to how important the checklist developer thought the bias might affect the results. Generally, the weightings of checklist questions have been subjectively applied with little if any empirical support, and subsequently total scores across checklists can be quite different. 6 Where weighting has not been applied across questions, the assumption is that all issues are of the same value and that is arguably not so. In light of these potential issues, at the Cochrane Collaboration Higgins et al 14 have indicated that readers refrain from giving an overall score to a paper on its methodological quality, but rather to identify whether methodological quality criteria have been met or not met, and in the latter case, how relevant the issue might be to the size of the effects observed in the study. This strategy makes it much harder for an individual to discern whether a particular paper is one that should be given more or less consideration, in respect to clinical decisions to be made. If clinicians are expected to assess the merits of individual experimental papers, this is an area that must be addressed further for more types of studies. Key sources of questionnaires for assessing the quality of intervention, risk factor and diagnostic studies are provided by Higgins et al, 14 Hayden et al, 15 Bossuyt et al, 16 and Whiting et al, 17 respectively.

APPLYING WHAT IS FOUND IN THE LITERATURE TO CLINICAL SCENARIOS

Assuming that papers have been identified that perform well from a methodological perspective, and their designs are well placed on the hierarchy for answering a particular question, finding papers that include participants who are similar to the patient(s) of interest to the professional is important. Such consideration should include an assessment of the level of severity of the groups under study (eg: mildly, moderately or severely affected), together with the amount of treatment they were being given, and the timing of that treatment within their disease/injury healing process. Furthermore, check when the researchers made their assessments to determine change in the participant's status. Ask whether these are realistic time points to do an assessment, and if the follow up was appropriate to determine the longer‐term effects.

It is also important that clinicians look beyond the treatment effect of an intervention to get a balanced view of its merits. Consideration should be made not only of the benefits but also the potential harm associated with a particular treatment. For instance, a new regime for treating acute muscle tears might be developed and shown in a well‐conducted RCT to allow players to return to sports much earlier than anything currently available. However, that same regime may induce side effects, perhaps a greater likelihood of the injury recurring 6‐12 months later due to the laying down of excessive scar tissue in the early stages of the rehabilitation regime. Examination of such points will allow the professional to make a better judgement concerning the relevance of the papers to the clinical decision at hand.

GUIDELINES PROVIDE A SYSTEMATIC REVIEW AND A SET OF RECOMMENDATIONS

Based on the information presented above, it would seem a monumental task for therapists to assess a series of individual papers and thereafter make an informed decision concerning every clinical problem that they face, particularly those where the patient is atypical, and does not resemble the subjects presented in studies. To make the task easier, guidelines have been developed to answer specific clinical problems/questions and provide recommendations. Because of the resources required, guidelines are usually initiated by organisations such as specialist groups in a field of medicine/allied health or a national health agency. These organisations convene a guidelines panel that is usually composed of scientists, clinical specialists, statisticians, patients and lay people, and they are supported by data analysts and administrators. Their first step is to identify the question of interest and the key outcomes associated with that question. They then assess systematic reviews (Level 1 evidence in the hierarchy) that have been previously published or specifically undertake their own systematic review. In doing so, they provide a summary of the quality of the research undertaken, the consistency of the results across studies, the magnitudes of the intervention's effects observed in patient subgroups, the benefits versus the potential harm associated with a treatment, and whether the health benefits of a treatment are worth the costs of providing them. 18 Most importantly though, guidelines include recommendations and these are often quite definitive, being categorised as ‘strong’ or ‘weak’. Guyatt et al 19 describe these as reflecting a trade off between the benefits of treatment against the burdens of receiving it together with its risks; while taking into account the accuracy and strength of the data supporting the intervention. If the data analysed from experimental papers indicates that an intervention has a large effect and the risks and burdens associated with the treatment are low, then a strong recommendation can be made to implement it. Where there are inconsistencies in findings or small treatment effects or notable risks, the recommendation for the treatment/intervention might be regarded as ‘weak’, and the patient's particular circumstances may then play a greater role in whether a particular treatment is implemented.

Given the extent and thoroughness behind the construction of guidelines and the inclusion of recommendations, they are an important source for guiding clinical decision making and should be searched for early in your examination of the literature.

THINK BEYOND THE SCIENCE

While the current paper has focused upon the quantitative assessment of evidence, it cannot be regarded as the sole means by which professionals make clinical decisions. It is important that therapists continue to appreciate the individuality of each patient and the personal circumstances that they bring with their pathophysiological issues. While at present, qualitative research does not have a formal place in levels of evidence, there is without doubt evidence for its importance in providing insights into patients' viewpoints on how the clinical condition and its treatment has influenced the lives that they lead.

Therefore, professionals must continue to value highly how we interact and react to each patient's situation, continually striving to be effective listeners and communicators, as well as being advocates of the best research evidence to help all patients improve the quality of their lives.

Key Points summary.

• The Pathway
• Best practice statements
• About the best practice statements

NHMRC Levels of Evidence

• Abbreviations and Definitions

For each statement, the primary reference has been graded according the NHMRC Levels of Evidence.

NHMRC levels of evidence were chosen as the NHMRC is the major funding body of the CCRE in Aphasia Rehabilitation and the levels align with the Australian Clinical Guidelines for Stroke Management (NSF, 2010).

Explanatory notes

• Definitions of these study designs are provided on pages 7-8 How to use the evidence: assessment and application of scientific evidence (NHMRC 2000b).
• The dimensions of evidence apply only to studies of diagnostic accuracy. To assess the effectiveness of a diagnostic test there also needs to be a consideration of the impact of the test on patient management and health outcomes (Medical Services Advisory Committee 2005, Sackett and Haynes 2002).
• If it is possible and/or ethical to determine a causal relationship using experimental evidence, then the ‘Intervention’ hierarchy of evidence should be utilised. If it is only possible and/or ethical to determine a causal relationship using observational evidence (ie. cannot allocate groups to a potential harmful exposure, such as nuclear radiation), then the ‘Aetiology’ hierarchy of evidence should be utilised.
• A systematic review will only be assigned a level of evidence as high as the studies it contains, excepting where those studies are of level II evidence. Systematic reviews of level II evidence provide more data than the individual studies and any meta-analyses will increase the precision of the overall results, reducing the likelihood that the results are affected by chance. Systematic reviews of lower level evidence present results of likely poor internal validity and thus are rated on the likelihood that the results have been affected by bias, rather than whether the systematic review itself is of good quality. Systematic review quality should be assessed separately. A systematic review should consist of at least two studies. In systematic reviews that include different study designs, the overall level of evidence should relate to each individual outcome/result, as different studies (and study designs) might contribute to each different outcome.
• The validity of the reference standard should be determined in the context of the disease under review. Criteria for determining the validity of the reference standard should be pre-specified. This can include the choice of the reference standard(s) and its timing in relation to the index test. The validity of the reference standard can be determined through quality appraisal of the study (Whiting et al 2003).
• Well-designed population based case-control studies (eg. population based screening studies where test accuracy is assessed on all cases, with a random sample of controls) do capture a population with a representative spectrum of disease and thus fulfil the requirements for a valid assembly of patients. However, in some cases the population assembled is not representative of the use of the test in practice. In diagnostic case-control studies a selected sample of patients already known to have the disease are compared with a separate group of normal/healthy people known to be free of the disease. In this situation patients with borderline or mild expressions of the disease, and conditions mimicking the disease are excluded, which can lead to exaggeration of both sensitivity and specificity. This is called spectrum bias or spectrum effect because the spectrum of study participants will not be representative of patients seen in practice (Mulherin and Miller 2002).
• At study inception the cohort is either non-diseased or all at the same stage of the disease. A randomised controlled trial with persons either non-diseased or at the same stage of the disease in both arms of the trial would also meet the criterion for this level of evidence.
• All or none of the people with the risk factor(s) experience the outcome; and the data arises from an unselected or representative case series which provides an unbiased representation of the prognostic effect. For example, no smallpox develops in the absence of the specific virus; and clear proof of the causal link has come from the disappearance of small pox after large-scale vaccination.
• This also includes controlled before-and-after (pre-test/post-test) studies, as well as adjusted indirect comparisons (ie. utilise A vs B and B vs C, to determine A vs C with statistical adjustment for B).
• Comparing single arm studies ie. case series from two studies. This would also include unadjusted indirect comparisons (ie. utilise A vs B and B vs C, to determine A vs C but where there is no statistical adjustment for B).
• Studies of diagnostic yield provide the yield of diagnosed patients, as determined by an index test, without confirmation of the accuracy of this diagnosis by a reference standard. These may be the only alternative when there is no reliable reference standard.

Note A: Assessment of comparative harms/safety should occur according to the hierarchy presented for each of the research questions, with the proviso that this assessment occurs within the context of the topic being assessed. Some harms are rare and cannot feasibly be captured within randomised controlled trials; physical harms and psychological harms may need to be addressed by different study designs; harms from diagnostic testing include the likelihood of false positive and false negative results; harms from screening include the likelihood of false alarm and false reassurance results.

Source: Hierarchies adapted and modified from: NHMRC 1999; Bandolier 1999; Lijmer et al. 1999; Phillips et al. 2001.

National Health and Medical Research Council. Additional levels of evidence and grades for recommendations for developers of guidelines 2008-2010

GET  IN  TOUCH

RESEARCH PARTNERS

© 2014 Australian Aphasia Rehabilitation Pathway Site terms of use   --  Privacy policy

NHMRC and NICE levels of evidence 

Contexts in source publication

Similar publications.

• PSYCHIAT PSYCHOL LAW
• Rosemary Williams

• Int J Environ Res Publ Health

• Nicole Hill

• PSYCHIAT SERV

• Terrence Haines
• Int J Ment Health Nurs

• AUSTRALAS PSYCHIATRY

• J PSYCHIATR MENT HLT

• Recruit researchers
• Join for free
• Login Email Tip: Most researchers use their institutional email address as their ResearchGate login Password Forgot password? Keep me logged in Log in or Continue with Google Welcome back! Please log in. Email · Hint Tip: Most researchers use their institutional email address as their ResearchGate login Password Forgot password? Keep me logged in Log in or Continue with Google No account? Sign up

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings
• My Bibliography
• Collections
• Citation manager

Save citation to file

Email citation, add to collections.

• Create a new collection
• Add to an existing collection

Add to My Bibliography

Your saved search, create a file for external citation management software, your rss feed.

• Search in PubMed
• Search in NLM Catalog
• Add to Search

Extending an evidence hierarchy to include topics other than treatment: revising the Australian 'levels of evidence'

Affiliation.

• 1 Adelaide Health Technology Assessment (AHTA), Discipline of Public Health, University of Adelaide, South Australia, Australia. [email protected]
• PMID: 19519887
• PMCID: PMC2700132
• DOI: 10.1186/1471-2288-9-34

Background: In 1999 a four-level hierarchy of evidence was promoted by the National Health and Medical Research Council in Australia. The primary purpose of this hierarchy was to assist with clinical practice guideline development, although it was co-opted for use in systematic literature reviews and health technology assessments. In this hierarchy interventional study designs were ranked according to the likelihood that bias had been eliminated and thus it was not ideal to assess studies that addressed other types of clinical questions. This paper reports on the revision and extension of this evidence hierarchy to enable broader use within existing evidence assessment systems.

Methods: A working party identified and assessed empirical evidence, and used a commissioned review of existing evidence assessment schema, to support decision-making regarding revision of the hierarchy. The aim was to retain the existing evidence levels I-IV but increase their relevance for assessing the quality of individual diagnostic accuracy, prognostic, aetiologic and screening studies. Comprehensive public consultation was undertaken and the revised hierarchy was piloted by individual health technology assessment agencies and clinical practice guideline developers. After two and a half years, the hierarchy was again revised and commenced a further 18 month pilot period.

Results: A suitable framework was identified upon which to model the revision. Consistency was maintained in the hierarchy of "levels of evidence" across all types of clinical questions; empirical evidence was used to support the relationship between study design and ranking in the hierarchy wherever possible; and systematic reviews of lower level studies were themselves ascribed a ranking. The impact of ethics on the hierarchy of study designs was acknowledged in the framework, along with a consideration of how harms should be assessed.

Conclusion: The revised evidence hierarchy is now widely used and provides a common standard against which to initially judge the likelihood of bias in individual studies evaluating interventional, diagnostic accuracy, prognostic, aetiologic or screening topics. Detailed quality appraisal of these individual studies, as well as grading of the body of evidence to answer each clinical, research or policy question, can then be undertaken as required.

PubMed Disclaimer

Similar articles

• The future of Cochrane Neonatal. Soll RF, Ovelman C, McGuire W. Soll RF, et al. Early Hum Dev. 2020 Nov;150:105191. doi: 10.1016/j.earlhumdev.2020.105191. Epub 2020 Sep 12. Early Hum Dev. 2020. PMID: 33036834
• [Procedures and methods of benefit assessments for medicines in Germany]. Bekkering GE, Kleijnen J. Bekkering GE, et al. Dtsch Med Wochenschr. 2008 Dec;133 Suppl 7:S225-46. doi: 10.1055/s-0028-1100954. Epub 2008 Nov 25. Dtsch Med Wochenschr. 2008. PMID: 19034813 German.
• Procedures and methods of benefit assessments for medicines in Germany. Bekkering GE, Kleijnen J. Bekkering GE, et al. Eur J Health Econ. 2008 Nov;9 Suppl 1:5-29. doi: 10.1007/s10198-008-0122-5. Eur J Health Econ. 2008. PMID: 18987905
• Recommendations from the international evidence-based guideline for the assessment and management of polycystic ovary syndrome. Teede HJ, Misso ML, Costello MF, Dokras A, Laven J, Moran L, Piltonen T, Norman RJ; International PCOS Network. Teede HJ, et al. Fertil Steril. 2018 Aug;110(3):364-379. doi: 10.1016/j.fertnstert.2018.05.004. Epub 2018 Jul 19. Fertil Steril. 2018. PMID: 30033227 Free PMC article. Review.
• Exploratory Examination of the Need for Revision of the DMP "Coronary Heart Disease" [Internet]. Institute for Quality and Efficiency in Health Care. Institute for Quality and Efficiency in Health Care. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2016 Feb 18. Extract of Rapid Report No. V15-01. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2016 Feb 18. Extract of Rapid Report No. V15-01. PMID: 29144635 Free Books & Documents. Review.
• Ten Ways to Improve Getting a Scientific Manuscript Accepted. Thompson LDR. Thompson LDR. Head Neck Pathol. 2024 Mar 19;18(1):22. doi: 10.1007/s12105-024-01617-6. Head Neck Pathol. 2024. PMID: 38503984
• Effectiveness of the Eutectic Mixture of Local Anaesthetics Cream in the Management of Arteriovenous Fistula Needle Insertion Pain in Patients Undergoing Haemodialysis. Shahri HA, Salmi IA, Rajhi WA, Shemshaki H. Shahri HA, et al. Sultan Qaboos Univ Med J. 2024 Feb;24(1):7-19. doi: 10.18295/squmj.10.2023.058. Epub 2024 Feb 28. Sultan Qaboos Univ Med J. 2024. PMID: 38434453 Free PMC article. Review.
• Mapping Effectiveness Studies of Occupational Therapy in Africa: A Scoping Review. Plastow NA, de Wit M, Brown M, de Kock M, Pretorius P, Pienaar S, Venter W. Plastow NA, et al. Occup Ther Int. 2023 Nov 22;2023:6688222. doi: 10.1155/2023/6688222. eCollection 2023. Occup Ther Int. 2023. PMID: 38034943 Free PMC article. Review.
• Delivery of telehealth nutrition and physical activity interventions to adults living in rural areas: a scoping review. Herbert J, Schumacher T, Brown LJ, Clarke ED, Collins CE. Herbert J, et al. Int J Behav Nutr Phys Act. 2023 Sep 15;20(1):110. doi: 10.1186/s12966-023-01505-2. Int J Behav Nutr Phys Act. 2023. PMID: 37715234 Free PMC article. Review.
• Development and Application of the Scale-Up Reflection Guide (SRG). Lee K, Crane M, Grunseit A, O'Hara B, Milat A, Wolfenden L, Bauman A, van Nassau F. Lee K, et al. Int J Environ Res Public Health. 2023 May 31;20(11):6014. doi: 10.3390/ijerph20116014. Int J Environ Res Public Health. 2023. PMID: 37297618 Free PMC article.
• NHMRC. A guide to the development, implementation and evaluation of clinical practice guidelines. Canberra, ACT: National Health and Medical Research Council, Commonwealth of Australia; 1999.
• NHMRC. How to review the evidence: systematic identification and review of the scientific literature. Canberra: National Health and Medical Research Council; 2000.
• Middleton P, Tooher R, Salisbury J, Coleman K, Norris S, Grimmer K, Hillier S. Corroboree: Melbourne. XIII Cochrane Colloquium, 22–26 October 2005. Melbourne: Australasian Cochrane Centre; 2005. Assessing the body of evidence and grading recommendations in evidence-based clinical practice guidelines.
• NHMRC additional levels of evidence and grades for recommendations for developers of guidelines. Stage 2 consultation. Early 2008 – end June 2009. http://www.nhmrc.gov.au/guidelines/consult/consultations/add_levels_grad...
• NHMRC. How to present the evidence for consumers: preparation of consumer publications. Canberra: National Health and Medical Research Council; 1999.
• Search in MeSH

Related information

• Cited in Books

LinkOut - more resources

Full text sources.

• BioMed Central
• Europe PubMed Central
• PubMed Central

• Citation Manager

NCBI Literature Resources

MeSH PMC Bookshelf Disclaimer

The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Unauthorized use of these marks is strictly prohibited.

• Corpus ID: 148442832

Table B.9, NHMRC Evidence Hierarchy: designations of ‘levels of evidence’ according to type of research question (including explanatory notes)

• P. Shekelle , M. Maglione , +2 authors Tanja Perry
• Published 2013

Tables from this paper

9 Citations

Guidelines versus evidence: what we can learn from the australian guideline for low-level laser therapy in knee osteoarthritis a narrative review, study protocol for a mixed methods prospective cohort study to explore experiences of care following a suicidal crisis in the australian healthcare system, restricting supply of tobacco products to pharmacies: a scoping review, review of reliable and valid noninvasive tools for the diagnosis of chronic exertional compartment syndrome, common themes and emerging trends for the use of technology to support mental health and psychosocial well-being in limited resource settings: a review of the literature, guidance for providing effective feedback in clinical supervision in postgraduate medical education: a systematic review, acetylcholinesterase inhibitors: beneficial effects on comorbidities in patients with alzheimer’s disease, association between alcohol restriction policies and rates of alcohol-related harms in remote australian aboriginal and torres strait islander communities: a systematic review.

• Highly Influenced

Interventions to Improve Gait in Older Adults with Cognitive Impairment: A Systematic Review

11 references, bmc medical research methodology open access extending an evidence hierarchy to include topics other than treatment: revising the australian 'levels of evidence', empirical evidence of design-related bias in studies of diagnostic tests., the architecture of diagnostic research, how to use the evidence: assessment and application of scientific evidence, bmc medical research methodology open access the relationship between quality of research and citation frequency, related papers.

Showing 1 through 3 of 0 Related Papers

NIMH Website Maintenance

Our website is currently undergoing required maintenance and will be temporarily unavailable. The website should be back online by 8:00 p.m. ET on Sunday, July 21st.

We apologize for any inconvenience and appreciate your patience.