Hierarchy of evidence
A hierarchy of evidence, comprising levels of evidence (LOEs), that is, evidence levels (ELs), is a heuristic used to rank the relative strength of results obtained from experimental research, especially medical research. There is broad agreement on the relative strength of large-scale, epidemiological studies. More than 80 different hierarchies have been proposed for assessing medical evidence.[1] The design of the study (such as a case report for an individual patient or a blinded randomized controlled trial) and the endpoints measured (such as survival or quality of life) affect the strength of the evidence. In clinical research, the best evidence for treatment efficacy is mainly from meta-analyses of randomized controlled trials (RCTs).[2][3] Systematic reviews of completed, high-quality randomized controlled trials – such as those published by the Cochrane Collaboration – rank the same as systematic review of completed high-quality observational studies in regard to the study of side effects.[4] Evidence hierarchies are often applied in evidence-based practices and are integral to evidence-based medicine (EBM).
Definition[edit]
In 2014, Jacob Stegenga defined a hierarchy of evidence as "rank-ordering of kinds of methods according to the potential for that method to suffer from systematic bias". At the top of the hierarchy is a method with the most freedom from systemic bias or best internal validity relative to the tested medical intervention's hypothesized efficacy.[5]: 313
In 1997, Greenhalgh suggested it was "the relative weight carried by the different types of primary study when making decisions about clinical interventions".[6]
The National Cancer Institute defines levels of evidence as "a ranking system used to describe the strength of the results measured in a clinical trial or research study. The design of the study ... and the endpoints measured ... affect the strength of the evidence."[7]
Criticism[edit]
In 2011, a systematic review of the critical literature found three kinds of criticism: procedural aspects of EBM (especially from Cartwright, Worrall and Howick),[34] greater than expected fallibility of EBM (Ioaanidis and others), and EBM being incomplete as a philosophy of science (Ashcroft and others).[35] Rawlins[36] and Bluhm note, that EBM limits the ability of research results to inform the care of individual patients, and that to understand the causes of diseases both population-level and laboratory research are necessary. EBM hierarchy of evidence does not take into account research on the safety and efficacy of medical interventions. RCTs should be designed "to elucidate within-group variability, which can only be done if the hierarchy of evidence is replaced by a network that takes into account the relationship between epidemiological and laboratory research"[37]
The hierarchy of evidence produced by a study design has been questioned, because guidelines have "failed to properly define key terms, weight the merits of certain non-randomized controlled trials, and employ a comprehensive list of study design limitations".[38]
Stegenga has criticized specifically that meta-analyses are placed at the top of such hierarchies.[39] The assumption that RCTs ought to be necessarily near the top of such hierarchies has been criticized by Worrall[40] and Cartwright.[41]
In 2005, Ross Upshur said that EBM claims to be a normative guide to being a better physician, but is not a philosophical doctrine.[42]
Borgerson in 2009 wrote that the justifications for the hierarchy levels are not absolute and do not epistemically justify them, but that "medical researchers should pay closer attention to social mechanisms for managing pervasive biases".[43] La Caze noted that basic science resides on the lower tiers of EBM though it "plays a role in specifying experiments, but also analysing and interpreting the data."[44]
Concato said in 2004, that it allowed RCTs too much authority and that not all research questions could be answered through RCTs, either because of practical or because of ethical issues. Even when evidence is available from high-quality RCTs, evidence from other study types may still be relevant.[45] Stegenga opined that evidence assessment schemes are unreasonably constraining and less informative than other schemes now available.[5]
In his 2015 PhD Thesis dedicated to the study of the various hierarchies of evidence in medicine, Christopher J Blunt concludes that although modest interpretations such as those offered by La Caze's model, conditional hierarchies like GRADE, and heuristic approaches as defended by Howick et al all survive previous philosophical criticism, he argues that modest interpretations are so weak they are unhelpful for clinical practice. For example, "GRADE and similar conditional models omit clinically relevant information, such as information about variation in treatments' effects and the causes of different responses to therapy; and that heuristic approaches lack the necessary empirical support". Blunt further concludes that "hierarchies are a poor basis for the application of evidence in clinical practice", since the core assumptions behind hierarchies of evidence, that "information about average treatment effects backed by high-quality evidence can justify strong recommendations", is untenable, and hence the evidence from individuals studies should be appraised in isolation.[46]
This article incorporates public domain material from Dictionary of Cancer Terms. U.S. National Cancer Institute.