Important progress during the past century, but plenty of scope for doing better

C ausal inferences about the effects of treatments must always depend on best judgments. Because the lives and wellbeing of patients will be influenced for better or worse by the validity of these judgments, however, it is important to be explicit about the logic as well as the empirical evidence on which the judgments are based. This issue of the BMJ is about one important aspect of that logic—the attempt to control bias through randomisation.

There is a growing acceptance that it is logical to try to control biases of various kinds when assessing the effects of treatments. Efforts by clinicians to control biases stretch back for at least three centuries,1 but only during the past 100 years have these become widespread. In particular, as we approach the end of the 20th century, there are now hundreds of thousands of reports of studies in which efforts have been made to control selection biases, the aim here being to distinguish differences attributable to treatments from differences that reflect the characteristics (known and unknown) of the people who have received treatment.

These studies are known as randomised trials because eligible patients are allocated at random to one of two or more alternative forms of care. This …