Pitfalls and biases in the reporting and interpretation of the results of clinical trials.

There are numerous biases (not all statistical) that can occur in the reporting and interpretation of the results of clinical trials. In this paper we review some of the major sources of such biases and propose some solutions for these problems. In particular, we review the biases that can occur in non-randomised studies; the effects of exclusion of patients after randomisation; reporting on the subset of patients receiving 'full' treatment only; comparing responders with non-responders; and emphasising secondary endpoints or subgroups. We also discuss the correct interpretation of a P-value and show the need for both estimates of the treatment effect and confidence intervals for reliable interpretation of the results. Finally, we consider the important but less well discussed problems of the interpretation of both classically 'negative' and 'positive' trials and the impact of the early stopping of the trial on the estimate of the treatment effect.