Hierarchy of evidence

A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. There is broad agreement on the relative strength of the principal types of epidemiological studies but no single, universally-accepted hierarchy of evidence. More than 80 different hierarchies have been proposed for assessing medical evidence. 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. Typically, systematic reviews and meta-analysies rank most highly, randomized controlled trials (RCTs) rank above observational studies, while expert opinion and anecdotal experience are ranked at the bottom. Evidence hierarchies are often applied in evidence-based practices and are integral to evidence-based medicine (EBM). A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. There is broad agreement on the relative strength of the principal types of epidemiological studies but no single, universally-accepted hierarchy of evidence. More than 80 different hierarchies have been proposed for assessing medical evidence. 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. Typically, systematic reviews and meta-analysies rank most highly, randomized controlled trials (RCTs) rank above observational studies, while expert opinion and anecdotal experience are ranked at the bottom. Evidence hierarchies are often applied in evidence-based practices and are integral to evidence-based medicine (EBM). In 2014, 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.:313 In 1997, Greenhalgh suggested it was 'the relative weight carried by the different types of primary study when making decisions about clinical interventions'. 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.' The term was first used in a 1979 report by the 'Canadian Task Force on the Periodic Health Examination' (CTF) to 'grade the effectiveness of an intervention according to the quality of evidence obtained'.:1195 The task force used three levels, subdividing level II: The CTF graded their recommendations into a 5-point A–E scale: A: Good level of evidence for the recommendation to consider a condition, B: Fair level of evidence for the recommendation to consider a condition, C: Poor level of evidence for the recommendation to consider a condition, D: Fair level evidence for the recommendation to exclude the condition, and E: Good level of evidence for the recommendation to exclude condition from consideration.:1195The CTF updated their report in 1984, in 1986 and 1987. In 1988, the United States Preventive Services Task Force (USPSTF) came out with its guidelines based on the CTF using the same 3 levels, further subdividing level II. Over the years many more grading systems have been described. In September 2000, the Oxford (UK) CEBM Levels of Evidence published its guidelines for 'Levels' of evidence re claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening. It not only addressed therapy and prevention, but also diagnostic tests, prognostic markers, or harm. The original CEBM Levels was first released for Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. As published in 2009 they are: In 2011, an international team redesigned the Oxford CEBM Levels to make it more understandable and to take into account recent developments in evidence ranking schemes. The Levels have been used by patients, clinicians and also to develop clinical guidelines including recommendations for the optimal use of phototherapy and topical therapy in psoriasis and guidelines for the use of the BCLC staging system for diagnosing and monitoring hepatocellular carcinoma in Canada.