Systematic reviews of complex interventions: framing the review question

The first and most important decision in preparing any systematic review is to clearly frame the question the review team seeks to answer. However, this is not always straightforward, particularly if synthesis teams are interested in the effects of complex interventions. In this article, we discuss how to formulate good systematic review questions of complex interventions. We describe the rationale for developing well-formulated review questions and review the existing guidance on formulating review questions. We discuss that complex interventions can contain a mix of effective and ineffective (or even harmful) actions, which may interact synergistically or dysynergistically or be interdependent, and how these interactions and interdependencies need to be considered when formulating systematic review questions. We discuss complexity specifically in terms of how it relates to the type of question, the scope of the review (i.e., lumping vs. splitting debate), and specification of the intervention. We offer several recommendations to assist review authors in developing a definition for their complex intervention of interest, which is an essential first step in formulating the review question. We end by identifying areas in which future methodological research aimed at improving question formulation, especially as it relates to complex interventions, is needed.