What are the components of complex interventions in healthcare? Theorizing approaches to parts, powers and the whole intervention.

The components of complex interventions are frequently discussed, invoked and examined in theory and research but seldom defined. This leads to theoretical and ontological ambiguities, lack of methodological transparency, and potentially, resistance to the wider movement towards complex intervention. This paper is the first to compare and contrast the different approaches that can be taken to the components of complex interventions. Most basically, complex interventions are defined as being composed of parts that make the whole intervention and, in isolation or combination, can generate the power of the intervention. Examples from the field of cardiac rehabilitation are used to illustrate key points. In relation to complex interventions past approaches variously: downplay complexity, focus on the complicatedness of complex interventions, or emphasize the complexity of complex interventions. Thus, approaches can be categorized as viewing components variously as: (1) Non existent parts and powers; (2) Irrelevant parts and powers; (3) Undifferentiated powerful parts; (4) Higher order parts and non-existent lower parts; (5) Higher order parts with non-powerful lower order parts; (6) Higher and lower order parts with powers; and (7) Components as the parts and the whole with powers. Based on this overview, complex interventions should be defined as being formed of parts, which can be material, human, theoretical, social, or procedural in nature, possibly stratified into higher and lower realms, that exercise power individually, in combination, or as emergent properties.