Distribution of combined p-values for baseline variables in a consecutive sample of randomized controlled trials

Background Combined p-values derived from summary statistics of baseline variables in randomized controlled trials (RCTs) are expected to follow a uniform distribution. Extreme values indicate similarity or dissimilarity across randomized groups. To date, investigations have been limited to a small number of clinical fields; here, we assess the distribution of combined p-values in a consecutive sample of RCTs. Methods Based on a PubMed search between February 1 and 18, 2022, 568 RCTs were included. Baseline variables were analysed using Carlisle’s method, yielding one combined p-value per RCT. A threshold of combined p-values of p ≤ 0.05 or p ≥ 0.95 was applied. Differences in combined p-value distributions between subgroups were assessed using the two-sample Anderson-Darling (AD) test. Results 26.2% (149/568) of the included RCTs had combined p-values of p ≤ 0.05 or p ≥ 0.95, compared with the 10% expected by chance. Of these, 104 RCTs (18.3%) were found in the upper extreme tail of p ≥ 0.95 and 45 RCTs (7.9%) in the lower extreme tail of p ≤ 0.05. Smaller RCTs (N ≤ 100) had more extreme combined p-values than larger RCTs (N > 100) (AD test p = 0.001). RCTs with > 7 baseline variables had more extreme combined p-values than those with ≤ 7 variables (AD test p = 0.004). A sensitivity analysis showed that 20.6% of RCTs still had extreme combined p-values after controlling for reporting errors related to SE and SD. Another sensitivity analysis showed that some extreme results could be explained by assuming high correlations between baseline variables within an RCT. Conclusion A substantially higher proportion of RCTs in a consecutive sample exhibited extreme similarities or dissimilarities in baseline characteristics across groups than expected by chance (26.2% vs 10%), indicating problems with randomisation. Further investigation into the causes and implications of implausible baseline distributions in RCTs is warranted. Registered 2023-03-20 at https://www.researchweb.org/fourol/project/280487