Group sample sizes in nonregulated health care intervention trials described as randomized controlled trials were overly similar.

We evaluated whether sample sizes in different arms of two-arm parallel group randomized controlled trials of nonregulated interventions were systematically closer in size than would plausibly occur by chance if simple randomization had been applied. We searched PubMed for trials of nonregulated health care interventions that did not report using restricted randomization from journals in behavioral sciences and psychology, nursing, nutrition and dietetics, rehabilitation, and surgery. We emailed trial authors to clarify randomization procedures. We identified 148 nonregulated intervention trials that indicated they used simple randomization. Difference in trial arm sizes was smaller than would be predicted by chance if simple randomization had occurred in all 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 were similar and statistically significant (P < 0.001) for trials that were published in journals with impact factors ≥ 4 and 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 implemented as intended and reported fully and accurately.

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