Statistics notes: Treatment allocation in controlled trials: why randomise?

Since 1991 the BMJ has had a policy of not publishing trials that have not been properly randomised, except in rare cases where this can be justified.1 Why? The simplest approach to evaluating a new treatment is to compare a single group of patients given the new treatment with a group previously treated with an alternative treatment. Usually such studies compare two consecutive series of patients in the same hospital(s). This approach is seriously flawed. Problems will arise from the mixture of retrospective and prospective studies, and we can never satisfactorily eliminate possible biases due to other factors (apart from treatment) that may have changed over time. Sacks et al compared trials of the same treatments in which randomised or historical controls were used and found a consistent tendency for historically controlled trials to yield more optimistic results than randomised trials.2 The use of historical controls can be justified only in tightly controlled situations of relatively rare conditions, such …