Objective: To assess the methodologic quality of approaches used to allocate participants to comparison groups in randomized controlled trials from one medical specialty.
Design: Survey of published, parallel group randomized controlled trials.
Data sources: All 206 reports with allocation described as randomized from the 1990 and 1991 volumes of four journals of obstetrics and gynecology.
Main outcome measures: Direct and indirect measures of the adequacy of randomization and baseline comparisons.
Results: Only 32% of the reports described an adequate method for generating a sequence of random numbers, and only 23% contained information showing that steps had been taken to conceal assignment until the point of treatment allocation. A mere 9% described both sequence generation and allocation concealment. In reports of trials that had apparently used unrestricted randomization, the differences in sample sizes between treatment and control groups were much smaller than would be expected due to chance. In reports of trials in which hypothesis tests had been used to compare baseline characteristics, only 2% of reported test results were statistically significant, lower than the expected rate of 5%.
Conclusions: Proper randomization is required to generate unbiased comparison groups in controlled trials, yet the reports in these journals usually provided inadequate or unacceptable information on treatment allocation. Additional analyses suggest that nonrandom manipulation of comparison groups and selective reporting of baseline comparisons may have occurred.