Incorrectly analysing stratified and minimised trials may lead to wrongfully rejecting superiority of interventions.

It is with great interest that we read the report of Yoshida et al 1 on the effect of second-generation narrow band imaging compared to white light imaging on detecting early gastric cancer in high-risk patients. The trial was expertly designed with a large patient population and, although superiority of narrow band imaging could not be proven, has important implications for further research on this topic. However, a significant issue concerning the analyses attracted our interest and we would like to comment on it. The primary outcome, the difference in proportion of patients in whom early gastric cancer was diagnosed, failed to reach statistical significance (p=0.412). This difference in proportions was tested for significance using Fisher’s exact test. This might not have been the proper method for analysis, as patients in the study were randomised using minimisation with a random component, stratified by institution, age and indication of endoscopy. Imbalance of risk factors between treatment and control arms can occur by chance …