Guidance for deciding upon use of primary mixed methods studies in research synthesis: lessons learned in childhood trauma

When reviewing literature, mixed methods studies (MMS) are increasingly retrieved, yet it is unclear how they should be dealt with in a research synthesis. In this article we examine the inclusion of primary MMS in research synthesis, based on experiences with a meta-analysis (MA) and a qualitative evidence synthesis (QES) in childhood trauma. The aim is to offer guidance for reviewers in deciding upon the use of MMS. This review article examines (1) the qualitative component, (2) the quantitative component, as well as (3) the third component of combined yield for use in a MA, a QES or a mixed studies review. A systematic search for MMS in the field of childhood trauma from January 1980 to October 2011 resulted in twelve MMS. Eight qualitative components, six quantitative components and one combined yield could have been included in a MA or QES. Exclusion of qualitative components was due to insufficient quality. Quantitative components were excluded because sample sizes were too small, different outcome measures than ours were used or no adequate statistics were provided. Yield could not be included because it was absent, outside the scope or otherwise unspecified. Finally, we offer flow charts with clear steps to assist researchers in deciding upon the use of components of MMS. Our study demonstrates that MMS can cover new areas and therefore cannot be neglected in a research synthesis.