When are complex interventions 'complex'? When are simple interventions 'simple'?

The issue of complexity, and how one evaluates complex interventions, remains a key one for health service and public health researchers. The underlying question—what makes complex interventions complex?—is particularly interesting, and it has implications for the sorts of research we do. The MRC guidance1 provides one detailed answer: complexity resides (among other things) in the number of interacting components; the number and difficulty of behaviours required by those delivering or receiving the intervention; the number of groups or organizational levels targeted by the intervention; the number and variability of outcomes; and the degree of flexibility or tailoring of the intervention permitted. Other definitions also emphasize other aspects including the degree of flexibility and non-standardization which complex interventions are subject to; as explained by Hawe et al.2 (2004), complex interventions are non-standard, having different forms in different contexts, while still conforming to specific, theory driven processes. There are many other definitions of complex interventions. These tend frequently to emphasize that they have multiple interacting components, and non-linear causal pathways. …