Claims of equivalence in medical research: Are they supported by the evidence?

Background: Most clinical studies are done to show comparative superiority, but many reports now claim equivalence between the investigated entities. These assertions may not always be supported by the methods used and the re sults obtained. Purpose: To assess the justification and support for claims of clinical or therapeutic equivalence in medical journals. Data Sources: A search of MEDLINE for articles published from 1992 through 1996. Study Selection: From 1209 citations that contained the word equivalence in the title or abstract or contained the Medical Subject Heading therapeutic equivalency, we excluded 1121 studies reporting nonoriginal research, pure ly laboratory or other nonhuman research, and studies in which equivalence was not the main claim. The remaining 88 eligible papers were evaluated for five methodologic attributes. Data Synthesis: Only 45 (51%) of the 88 reports were specifically aimed at studying equivalence; the others either tried to show superiority or did no t state a research aim. The quantitative distinctions regarded as "equivale nt" ranged from 0% to 21% for direct increments and from 0% to 76% for prop ortionate differences. An equivalence boundary was set and confirmed with a n appropriate statistical test in only 23% of reports. In 67% of reports, e quivalence was declared after a failed test for comparative superiority, an d in 10%, the claim of equivalence was not statistically evaluated. The sam ple size needed to confirm results had been calculated in advance for only 33% of reports. Sample size was 20 patients per group or fewer in 25% of re ports. Conclusions: Many studies of clinical equivalence do not set boundaries for equivalence. Claims of "difference" or "similarity" are often made not by thoughtful examination of the data but by tests of statistical significance that are often misapplied or accompanied by inadequate sample sizes. These methodologic flaws can lead to false claims, inconsistencies, and harm to patients.