Analysis and comment Health services research Process evaluation in randomised controlled trials of complex interventions

“Complex interventions” are health service interventions that are not drugs or surgical procedures, but have many potential “active ingredients.” A complex intervention combines different components in a whole that is more than the sum of its parts. Randomised controlled trials (RCTs) are the most rigorous way to evaluate the effectiveness of interventions, regardless of their complexity. Because of their multifaceted nature and dependence on social context, complex interventions pose methodological challenges, and require adaptations to the standard design of such trials. This paper outlines a framework for using process evaluation as an integral element of RCTs. It draws on experience from a cluster randomised trial of peer led sex education.