Methodological reflections on using qualitative research to explore the causal mechanisms of complex health interventions

<jats:p> We reflect on how qualitative research can be used to develop or refine theories about how the mechanisms triggered by intervention enactment might generate outcomes, referring to examples from a ‘realist trial’ of a whole-school health intervention. Qualitative research can explore mechanisms directly, by asking participants how they think interventions work, or indirectly, by exploring participant experiences of intervention-related actions to understand the conditions and consequences of these actions. Both of these approaches can inform theorisation of how mechanisms are triggered and generate outcomes, and how this is contingent on context. We discuss methods for sampling, data collection and data analysis, and recommend dimensional analysis as a means to analyse qualitative data on mechanisms. We then consider how to draw on qualitative research to inform hypotheses to be tested statistically. </jats:p>