The influence of study characteristics on reporting of subgroup analyses in randomised controlled trials: systematic review
Randomised controlled trials published in high impact journals, with larger sample size, studying non-surgical topics, and with industry funding—if the primary outcome is not statistically significant—are associated with more frequent reporting of subgroup analyses. The proportion of trials prespecifying subgroup hypotheses and carrying out interaction tests for subgroup analyses is low in both industry funded and non-industry funded trials. Industry funded trials, regardless of the statistical significance of primary outcomes, less often prespecify subgroup hypotheses and less often use the interaction test for analyses of subgroup effects compared with trials that are not funded by industry. Our findings suggest that clinicians, reviewers, and journal editors should view all subgroup analyses with caution. Particular attention is warranted in industry funded trials with negative results for the primary outcome.