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
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:
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.
|Question Type or Domain||Best Research Design to answer the Question|
|Therapy (Treatment)||Randomised Contolled Trial (RCT)|
|Prevention||RCT or Prospective Study|
|Diagnosis||RCT or Cohort Study|
|Prognosis (Forecast)||Cohort Study and/or Case-Control Series|
|Etiology (Causation)||Cohort Study|
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.
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:
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.
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 PubMed.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). 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.