Hierarchy of evidence

[[File:Research design and evidence - Capho.svg|thumb|right|165px|A typical evidence pyramid used to describe a hierarchy of evidence]]
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A '''hierarchy of evidence''' is a ranking of the quality of [[evidence]] used in [[evidence-based medicine]]. There is variation among hierarchies, with more than 80 documented,<ref name=blunt1>[http://cjblunt.com/hierarchies-evidence/ Hierarchies of Evidence] Chris J. Blunt</ref> but they follow a basic pattern of attempting to rank evidence based on:
* Relevance to the target population (usually [[humans]], but potentially also [[animals]] in veterinary medicine), e.g. studies on humans are more relevant to humans than studies on rodents
* Throwing out bad studies entirely. This can be for lots of reasons, and is done in quality [[Review article|reviews]] and [[meta-analyses]]. Common reasons for ignoring studies include:
** poor design
** lack of [[peer review]]
** insufficient control for biases
** having research funding from a party (such as an industry) that has an interest in the outcome.<ref name=greenhalgh>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2127173/pdf/9253275.pdf ''How to read a paper'': Getting your bearings (deciding what the paper is about)] by Trisha Greenhalgh (1997) ''BMJ'' 315:243-315.</ref><ref name=belluz>[https://www.macleans.ca/uncategorized/the-oprah-effect-and-why-not-all-scientific-evidence-is-valuable/ The Oprah effect and why not all scientific evidence is valuable: Some studies are more equal than others] by Julia Belluz (Nov 9, 2011) ''Maclean's''.</ref>
:It is important to recognize and remove bad studies from reviews and meta-analyses (and it is important to keep in mind that bad studies can appear in peer-reviewed journals) because bad studies can taint the outcomes of the reviews/meta-analyses.<ref name=greenhalgh/> 
* [[Statistical]] power, e.g. all other things being equal, studies on more individuals have more statistical power (and can therefore potentially give results with greater [[statistical significance]]) than studies on fewer individuals 
* Predictive power, that the evidence shows [[Correlation does not imply causation| causation rather than just correlation]] — e.g., some epidemiological studies can show evidence of causation<ref>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1731812/pdf/v055p00905.pdf Causation in epidemiology] by M. Parascandola & D. Weed (2001) ''J. Epidemiol. Community Health'' 55(12): 905–912. doi: 10.1136/jech.55.12.905.</ref>

The Canadian Task Force on the Periodic Health Examination first described  an explicit hierarchy of evidence in 1979.<ref name=blunt1/><ref>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1704686/pdf/canmedaj01457-0037.pdf The Levels of Evidence and their role in Evidence-Based Medicine] by Patricia B. Burns et al. (2011) ''Plast. Reconstr. Surg.''128(1): 305–310. doi:10.1097/PRS.0b013e318219c171.</ref><ref>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1704686/pdf/canmedaj01457-0037.pdf The periodic health examination] by the Canadian Task Force on the Periodic Health Examination (1979) ''CMA Journal'' 121:1193-1254.</ref> Implicit hierarchies of evidence can be inferred from literature reviews that predate 1979, however, such as the monograph series of the International Agency for Research on Cancer (IARC) that began in 1972.<ref name=iarc>[https://monographs.iarc.fr/monographs-and-supplements-available-online/ IARC Monographs on the Evaluation of Carcinogenic Risk to Humans] International Agency for Research on Cancer</ref>

A generally-accepted hierarchy of evidence is:<ref name=greenhalgh/><ref>[https://jamanetwork.com/journals/jama/article-abstract/392650 Users' guides to the medical literature. IX. A method for grading health care recommendations] by G. H. Guyatt GH et la. (1995) ''JAMA'' 274:1800-4.</ref>
# Systematic reviews and meta-analyses
# [[Randomized controlled trial]]s with definitive results
# Randomized controlled trials with non-definitive results
# [[Cohort study|Cohort studies]]
# [[Case-control study|Case-control studies]]
# Cross-sectional surveys
# Case reports

IARC Monographs, particularly the more recent ones, use a hierarchy that resembles the following, where evidence at the bottom can be used to support evidence at the top for overall evaluations:<ref name=iarc/>
# systematic reviews and meta-analyses
# randomized controlled trials in humans
# cohort studies in humans
# case-control studies in humans
# chronic studies in animals
# acute studies in animals, ''in vitro'' studies, and mechanistic studies

==Criticisms==
{{cquote|Hierarchies are a poor basis for the application of evidence in clinical practice. The Evidence-Based Medicine movement should move beyond them and explore alternative tools for appraising the overall evidence for therapeutic claims.|||Christopher J. Blunt<ref name=blunt2>[http://etheses.lse.ac.uk/3284/ Hierarchies of evidence in evidence-based medicine] by Christopher J Blunt (2015) PhD thesis, The London School of Economics and Political Science.</ref>}}
Hierarchies of evidence are not without criticism, but the criticism has come from within the [[philosophy of science]] and has generally been ignored within evidence-based medicine.<ref>[https://www.researchgate.net/publication/7538076_Looking_for_Rules_in_a_World_of_Exceptions_reflections_on_evidence-based_practice Looking for Rules in a World of Exceptions: reflections on evidence-based practice] by Ross E. G. Upshur (2005) ''Perspect. Biol. Med.'' 48(4): 477-89.</ref> 

Problems with evidence hierarchies include:<ref name=blunt2/><ref>[https://www.sciencenews.org/blog/context/critique-medical-evidence-hierarchies Philosophical critique exposes flaws in medical evidence hierarchies: Rankings of research reliability are logically untenable, an in-depth analysis concludes] by Tom Siegfried (2:30pm, November 13, 2017) ''Science News''.</ref>
*There is no method for evaluating which of the many different hierarchies is better for a given problem.
*Hierarchies of evidence are at best a [[heuristic]] and lack both [[empirical]] and theoretical justification.
*Studies with high internal validity (well-controlled and with a homogeneous study group) might have low external validity for a heterogeneous population.

Although there are serious criticisms of the hierarchy of evidence in evidence-based medicine, there has been no serious proposal for replacing this type of methodology in dealing with prioritization of the sometimes massive amounts of evidence relevant to a given problem.

==References==
{{reflist|2|80%}}

[[Category:Medicine]]
[[Category:Philosophy of science]]
[[Category:Science]]
[[Category:Types of medical studies]]

Categories: Medicine, Philosophy of science, Science, Types of medical studies