Table 3.

Trials with adequately concealed allocation compared with inadequately concealed allocation

Study	Sample (search strategy)	Comparison	Results	Direction of bias
Chalmers 198318	145 controlled trials of treatment for acute myocardial infarction (systematic)	Studies with different allocation schemes (non-random, non-concealed random, and concealed random allocation) on maldistribution of prognostic variables, frequency of significant outcomes, and case fatality rates	In non-RCTs, non-concealed RCTs, and RCTs with concealed allocation, the maldistribution of prognostic factors was 34%, 7%, and 3.5% respectively, frequency of significant outcomes was 25%, 11%, and 5% respectively, average relative risk reduction for mortality was 33%, 23%, and 3% respectively. Case fatality rate for control groups was 32%, 23%, and 16% and for treatment groups was 21%, 18%, and 16% respectively	Overestimation of effect
Schulz 199519	250 RCTs from 33 meta-analyses (Cochrane Pregnancy and Childbirth Database)	Association between methodological features of controlled trials (allocation concealment, double blinding, and follow up), and treatment effect (odds ratio)	Treatment effect overestimated by 41% in RCTs with inadequate concealment and by 30% in RCTs with unclear adequacy of concealment compared with those with adequate concealment (P⩽0.001) after adjustment for other methodological features. Studies with no double blinding overestimated treatment effect by 17% compared with double blinded studies (P =0.01). Lack of complete follow up had no influence on treatment effect (7%, P=0.32)	Overestimation of effect

RCT=Randomised controlled trial.