Comparability work and the management of difference in research synthesis studies.

The new imperative to be more methodologically inclusive has generated a burgeoning interest in synthesizing the findings of qualitative and quantitative studies, or mixed research synthesis. Yet, the very diversity seen to define the mixed research synthesis enterprise is also considered to defy it as it intensifies the problem of comparing the seemingly incomparable to enable the combination of the seemingly uncombinable. We propose here that the research synthesis enterprise, in general, and the mixed research synthesis enterprise, in particular, entail comparability work whereby reviewers impose similarity and difference on the studies to be reviewed. The very study diversity requiring management does not exist a priori but rather is itself an outcome of comparability work already done whereby judgments have been made about what constitutes methodological and topical diversity and uniformity. Conceiving the research synthesis process as defined by comparability work moves the backstage interpretive work of systematic review to center stage and, thereby, sets a new stage for addressing the methodological issues involved. These issues are explored by reference to the synthesis of empirical studies of antiretroviral adherence in HIV-positive women in the US.