Identical summary statistics were uncommon in randomised trials and cohort studies.

OBJECTIVE To examine the proposition that identical summary statistics (mean and/or SD) in different randomised controlled trials (RCTs) or clinical cohorts can be explained by common or homogeneous source populations. STUDY DESIGN We estimated the probability of identical summary data in studies with high proportions of identical summary statistics, in simulations, and in control datasets. RESULTS The probability of both an identical mean and an identical SD for a variable in separate RCTs is low (<∼3%), unless the variable is rounded to 1 significant figure. In two RCTs with identical summary statistics for 16/39 shared variables, simulations indicated the probability of the observed matches was <1/100,000. In 34 clinical cohorts with publication integrity concerns, the proportion of summary statistics from variables reported in ≥10 studies that were identical in ≥2 cohorts was high (42% for means, 52% for SDs, and 29% for both), and improbable based on simulations and comparisons to control datasets. CONCLUSIONS The likelihood of multiple identical summary statistics within an individual RCT or across a body of RCTs or cohort studies by the same research group is low, especially when both the mean and the SD are identical, unless the variables are rounded to 1 significant figure.