Table 1.

Strengths and limitations of randomized controlled trials (RCTs) and non-randomized studies (NRSs)

	RCTs	NRSs
Theoretical
Certainty of the evidence regarding causality	Higher	Lower
Confounding	Unlikely for large RCTs	Adjustment for known and measured confounders possible; residual confounding likely
Levels of exposure	Few; often relatively high differences in intervention groups	Broad range, possibility of stratifying by exposure level
Follow-up time of study	Short or limited	Long
Empirical
Number of participants	Usually < 1000	Some > 10,000
Representativeness for general population	Often limited	Generally good
Outcome measures	Often risk factor, occasionally morbidity/ mortality	Usually morbidity/ mortality