Balance of group sizes in randomized controlled trials published in American Psychological Association journals

We evaluated whether sample size differences between arms of two-arm parallel group randomized controlled trials (RCTs) published in American Psychological Association (APA)-affiliated journals were consistently smaller than expected by chance with simple randomization.

We searched PsycINFO for two-arm parallel group RCTs in APA-affiliated journals published January 2007 to September 2017 that used individual randomization (1:1 allocation ratio), reported the number of participants randomized, and did not describe employing restrictive randomization (e.g., blocking). We queried authors because randomization processes were often not described in articles, and we conducted a post hoc logistic regression analysis to attempt to identify factors associated with overly balanced groups.

We identified 203 eligible trials, but after the author query, it was determined that only 115 used simple randomization. Among those 115 trials, there was a significantly greater number of trials with smaller sample size differences between trial arms than would be expected by chance (p < .001); 89 of 115 (77%) had differences in trial arm sample sizes smaller than the 50% prediction interval threshold for these differences. Greater proportionate imbalance may be associated with larger trial size (odds ratio of 0.27, 95% CI [0.08, 0.94] for N > 200 vs. N ≤ 100); greater balance may be more common in higher impact journals, though this was not statistically significant.

Education is needed to ensure that randomization procedures are implemented as intended and fully and accurately reported and that balanced group sample sizes are not understood as an indicator of trial quality. (PsycInfo Database Record (c) 2020 APA, all rights reserved).