Using qualitative comparative analysis (QCA) in systematic reviews of complex interventions: a worked example.

Background: Systematic reviews that address policy and practice questions in relation to complex interventions frequently need not only to assess the efficacy of a given intervention but to identify which intervention - and which intervention components - might be most effective in particular situations. Here, intervention replication is rare, and commonly used synthesis methods are less useful when the focus of analysis is the identification of those components of an intervention that are critical to its success.

Methods: Having identified initial theories of change in a previous analysis, we explore the potential of qualitative comparative analysis (QCA) to assist with complex syntheses through a worked example. Developed originally in the area of political science and historical sociology, a QCA aims to identify those configurations of participant, intervention and contextual characteristics that may be associated with a given outcome. Analysing studies in these terms facilitates the identification of necessary and sufficient conditions for the outcome to be obtained. Since QCA is predicated on the assumption that multiple pathways might lead to the same outcome and does not assume a linear additive model in terms of changes to a particular condition (that is, it can cope with 'tipping points' in complex interventions), it appears not to suffer from some of the limitations of the statistical methods often used in meta-analysis.

Results: The worked example shows how the QCA reveals that our initial theories of change were unable to distinguish between 'effective' and 'highly effective' interventions. Through the iterative QCA process, other intervention characteristics are identified that better explain the observed results.

Conclusions: QCA is a promising alternative (or adjunct), particularly to the standard fall-back of a 'narrative synthesis' when a quantitative synthesis is impossible, and should be considered when reviews are broad and heterogeneity is significant. There are very few examples of its use with systematic review data at present, and further methodological work is needed to establish optimal conditions for its use and to document process, practice, and reporting standards.