Dyson et al¹ use a pragmatic design to address an interesting question, but I am concerned that the statistical analysis may be inappropriate and could have led to erroneous conclusions being drawn. The study is a cluster randomised controlled trial. Instead of randomising individual house officers (HOs), the authors have randomised groups of HOs (those working at the same hospital). This is entirely appropriate. As the authors point out, randomising individual HOs would risk contamination between the two study groups by HOs sharing aide memoires.

However, if groups, rather than individuals, are randomised then the use of standard statistical techniques may be inappropriate. These techniques assume that all observations (that is, all individuals) are independent of each other. Yet in a cluster trial this may not be true. HOs at the same hospital are likely to share characteristics and …