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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.
[1] [2] Hierarchy of study design, for example using a case-study, ecological study, cross-sectional, case-control, cohort, or experimental, although not always in this order is a general rule to a high "strength of evidence" of a clinical study. [3] [4] [5]
Evidence-based medicine (EBM) is "the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. ...[It] means integrating individual clinical expertise with the best available external clinical evidence from systematic research."
Such evidence is expected to be empirical evidence and interpretable in accordance with the scientific method. Standards for scientific evidence vary according to the field of inquiry, but the strength of scientific evidence is generally based on the results of statistical analysis and the strength of scientific controls. [citation needed]
In scientific research evidence is accumulated through observations of phenomena that occur in the natural world, or which are created as experiments in a laboratory or other controlled conditions. Scientists tend to focus on how the data used during statistical inference are generated. [40]
A systematic review is a scholarly synthesis of the evidence on a clearly presented topic using critical methods to identify, define and assess research on the topic. [1] A systematic review extracts and interprets data from published studies on the topic (in the scientific literature), then analyzes, describes, critically appraises and summarizes interpretations into a refined evidence-based ...
Given their intrinsic methodological limitations, including lack of statistical sampling, case reports are placed at the bottom of the hierarchy of clinical evidence, together with case series. [3] Nevertheless, case reports do have genuinely useful roles in medical research and evidence-based medicine. [4]
Alternatively, one could group subjects based on their body mass index (BMI) and compare their risk of developing heart disease or cancer. Prospective cohort studies are typically ranked higher in the hierarchy of evidence than retrospective cohort studies [3] and can be more expensive than a case–control study. [4]