Notes on the use of randomised controlled trials to evaluate complex interventions: Community treatment orders as an illustrative case.

Over the past seven decades, randomised controlled trials (RCTs) have revolutionised clinical research and achieved a gold standard status. However, extending their use to evaluate complex interventions is problematic. In this paper we will demonstrate that complex intervention RCTs violate the necessary premises that govern the RCTs logic and underpin their rigour. The lack of blinding, heterogeneity of participants, as well as poor treatment standardisation and difficulty of controlling for confounders, which characterise complex intervention RCTs, can potentially be profoundly detrimental to their integrity. Proponents of this approach argue that matching "real world" circumstances, while maintaining the randomised design, enhances external validity. We counter this argument by pointing out that an inverted U relation exists between internal and external validity, and thus relaxing the experimental conditions beyond a certain threshold can potentially paradoxically render the RCT externally invalid, i.e. its results cannot be used anywhere. We shall illustrate the inappropriate use of RCTs to evaluate community treatment orders and propose an alternative epistemic model that is based on mechanistic reasoning and Cartwright's capacity concept.