Particularly when the results of a clinical research paper are conflicting or counter-intuitive, even an approximate estimate of its strength is valuable.
In general, the rarer the type of study, the greater its strength. The main types of clinical research in descending order of frequency are case reports, case series, case-control studies, cohort studies, randomised controlled trials, systematic reviews and meta-analyses1.
Strength of clinical evidence is currently graded as follows2:
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