Randomisation to protect against selection bias in healthcare trials

Randomised trials use the play of chance to assign participants to comparison groups. The unpredictability of the process, if not subverted, should prevent systematic differences between comparison groups (selection bias), provided that a sufficient number of people are randomised.

To assess the effects of randomisation and concealment of allocation on the results of healthcare trials.

We searched the Cochrane Methodology Register, MEDLINE, SciSearch, reference lists up to August 2000 and used personal communication.

Cohorts of trials, systematic reviews or meta-analyses of healthcare interventions that compared outcomes or prognostic factors for one of the following comparisons: randomised versus non-randomised trials, randomised trials with adequately versus inadequately concealed allocation, or high versus low quality trials where selection bias could not be separated from other sources of bias.

One of us went through all of the citations in the Cochrane Methodology Register and accumulated reference lists. Studies that appeared to meet the inclusion criteria were retrieved and assessed independently by two of the reviewers. The methodological quality of included studies was appraised and information extracted by one of us and checked by a second. Tabular summaries of the results were prepared for each comparison and the results across studies were assessed qualitatively to identify common trends or discrepancies.

We identified 32 studies including over 3000 trials. Twenty-two studies compared randomised versus non-randomised trials, three compared adequately versus inadequately concealed allocation, and nine compared high versus low quality trials (some studies included more than one comparison). Five studies were of high methodological quality. In 15 of the 22 studies that compared randomised and non-randomised trials of the same intervention, important differences were found in the estimates of effect. Some of these differences were due to a poorer prognosis in the control groups in the non-randomised trials. The results of the other seven studies that compared randomised and non-randomised trials across different interventions are less clear. Comparisons of adequately and inadequately concealed allocation in randomised trials of the same intervention provided high quality evidence that concealment can be crucial in achieving similar treatment groups and, therefore, unbiased estimates of treatment effects. Studies with inadequate concealment tended to overestimate treatment effects. Comparisons of high and low quality trials of the same intervention have found important differences in estimates of effect, but it is not possible to determine the extent to which these differences can be attributed to randomisation or concealment of allocation. Omitting comparisons between randomised trials and non-randomised trials using historical controls did not substantially alter the results or conclusions of our review.

On average, non-randomised trials and randomised trials with inadequate concealment of allocation tend to result in larger estimates of effect than randomised trials with adequately concealed allocation. However, it is not generally possible to predict the magnitude, or even the direction, of possible selection biases and consequent distortions of treatment effects.