A review of all published articles in the British Journal of Sports Medicine from 1991 to 1995¹ showed that a wide range of subjects had been covered, but that randomised controlled trials (RCTs) accounted for only 3% of the total number of articles. Common study designs for evaluating (exercise) interventions are “before and after” or “uncontrolled” trials. In these, outcomes of interest in subjects are measured before and after the intervention or exposure—for example, saliva tests before and after an exercise test, or the trials may be unmatched or matched case-control studies, in which subjects under exposure or measurement are compared with controls—for example, by comparison of a questionnaire survey completed by athletes and controls.

These designs, though valid and important, are open to potential bias. The scientific outsider may be sceptical about the results and conclusions of such studies in the absence of the authors' detailed description of—for example, recruitment strategies, inclusion criteria, loss to follow up, and rules for interpretation of results of the intervention. Perhaps more importantly, interventions like exercise programmes or advice on physical activity may seem to produce positive results in research projects, but fail to deliver them in real life because of bias hidden in the original study. This can leave practitioners sceptical about research claims and prone to reject a rigorous approach to the evaluation of new ideas and techniques. Minimising bias is important in making sure that we really know what works.

In this paper we try to indicate possible sources of bias in the design and analysis of experimental studies and remind sports scientists about the benefits of RCTs for reducing bias.