Objective: To discuss the extent to which the results of randomized controlled trials (RCTs) in psychiatry can be generalized to clinical practice.
Method: Threats to internal and external validity in psychiatric RCTs are reviewed.
Results: Threats to internal validity increase the possibility of bias. Psychiatric RCTs have problems with small samples, arbitrary definitions of caseness, disparate definitions of outcome and high spontaneous recovery rates. Particular issues arise in psychotherapy RCTs. Threats to external validity reduce the extent to which the results of a RCT produce a correct basis for generalization to other circumstances. These include high rates of comorbidity and sub syndromal pathology in normal clinical practice, manual-based treatment protocols and varying definitions of successful treatment.
Conclusions: Randomized controlled trials remain the most robust design to investigate the effectiveness of treatments. They should be applied to important clinical questions; and carried out, as far as possible, with typical patients in the clinical conditions in which the treatment is likely to be used.