This chapter reflects on how evidence from realist trials and systematic reviews might be of value, not only in drawing conclusions about specific interventions and their theories of change but also in testing and refining the middle range theories which inform these and other interventions. While evaluation evidence should be of most immediate use in informing decisions about the implementation of the specific interventions being evaluated, a broader and more enduring use for evaluation could be in suggesting refinements to middle range theory. Such refinements might then be used to inform and influence the next generation of complex health interventions. In order to be useful in assessing the validity of middle range theory, evaluations will need to assess interventions informed by a limited number of middle range theories comprising a limited number of well-defined constructs. There may be value in conducting proof of principle studies separately from more pragmatic evaluations in order to test and refine middle range theory.

Theories of change propose how intervention resources and activities might lead to the generation of outcomes. They are sometimes presented diagrammatically as logic models. Realist evaluators and others have suggested that interventions should be theorised in terms of how intervention mechanisms interact with context to generate outcomes. Our own trial of the Learning Together whole-school intervention to prevent bullying set out to define, refine and test such theories in the form of context–mechanism–outcome configurations (CMOCs). We drew on several sources to define our starting CMOCs. These included existing middle range theory. This is scientific theory about the general mechanisms (i.e. not necessarily concerning an intervention) that generate outcomes. This should be analytically general enough to apply to a range of settings, populations and/or outcomes, but specific enough to be useful in a given application. We also used previous research and public consultation to inform our CMOCs.