It is eight years since the publication of the Medical Research Council’s original report on methods for developing and evaluating randomised controlled trials for complex interventions.1 Although presented as a “discussion document,” the MRC framework and its companion paper have often been cited as authoritative guidance on methods. Other people, however, have found the definition of the complexity of interventions narrow and misconceived,2 and the suggested phases for developing and evaluating complex interventions as unhelpfully similar to commercial drug evaluation. However, the report can probably be credited with stimulating much of the ongoing debate about appropriate methods and concepts in healthcare evaluation—particularly when the intervention of interest is hard to define, hard to evaluate (using conventional experimental methods), or just hard to explain.

The MRC has now updated its original report (www.mrc.ac.uk/complexinterventionsguidance ) to reflect recent developments in methods and lessons learnt in applying them. The guidance is summarised in the linked article by Craig and colleagues (doi:10.1136/bmj.a1655).3 It has a broader scope than the original …