Group Sample Sizes in Non-Regulated Health Care Intervention Trials Described as Randomized Controlled Trials were Overly Similar.

•We examined whether sample sizes in different arms of two-arm parallel group RCTs of nonregulated interventions were systematically closer in size than would plausibly occur by chance if simple randomization had been applied.•We reviewed 148 trials of nonregulated interventions from behavioral science and psychology, nursing, nutrition and dietetics, rehabilitation, and surgery that were verified as having used simple randomization.•Differences in group sizes were significantly smaller than would be predicted by chance if simple randomization had actually occurred in all of the trials (P < 0.001). Rather than approximately half of the trials being within a 50% prediction interval for the difference, 96% had differences within this interval.•Results did not differ when only high-impact journals were evaluated or when stratified by type of nonregulated intervention.•There is a need for education and better understanding of clinical trial methods to ensure that randomization procedures are reported fully and accurately and that randomization occurs as intended.