Relative Citation Impact of Various Study Designs in the Health Sciences

Abstract

Several authors and organizations have proposed hierarchies of evidence, based on the relative reliability of various types of study designs.^1-4 Although many people recognize that expert opinions and nonsystematic reviews provide the least reliable level of information,^5,6 such articles continue to have a massive influential presence.⁷ Controlled studies assume higher places in hierarchies of evidence than uncontrolled studies, and randomized trials are considered the gold standard for clinical research.^1-4 However, randomized trials cannot be conducted for all questions of interest⁸ and there is debate on whether they give different results than nonrandomized studies.^9-14 Finally, meta-analyses are becoming increasingly frequent in the literature. Meta-analyses are often placed at the highest level of evidence,^1-4 despite their critics.^15,16 No hierarchy of evidence is unanimously accepted.