Interpreting the evidence: choosing between randomised and non-randomised studies

Treatment effects obtained from randomised and non-randomised studies may differ, but one method does not give a consistently greater effect than the other. Treatment effects measured in each type of study best approximate when the exclusion criteria are the same and where potential prognostic factors are well understood and controlled for in the non-randomised studies. Subjects excluded from randomised controlled trials tend to have a worse prognosis than those included, and this limits generalisability. Subjects participating in randomised controlled trials evaluating treatment of existing conditions tend to be less affluent, educated, and healthy than those who do not; the opposite is true for trials of preventive interventions.