Conclusions about medical interventions or the causes of disease are based upon reviews of the scientific literature. Single studies usually have limited statistical power or may be difficult to interpret or generalize and so the findings from a single study can rarely justify a change in clinical practice or in an aetiological theory. Even when planning larger studies or mega-trials (Yusuf et al. 1984), a thorough review of existing literature is needed and the results of the study need to be placed in that context, though single studies can exert an important and powerful influence.