Getting it wrong most of the time? Comparing trialists' choice of primary outcome with what patients and health professionals want.

The primary outcome in a trial is the most important piece of data collected. It is used to determine how many participants are required, and it is the main piece of information used to judge whether the intervention is effective or not. In our study, patients and healthcare professionals agreed with the choice of the primary outcome made by trial teams doing late-stage trials in breast cancer management and nephrology 28% of the time.