The contribution of qualitative evidence to health care decision-making is increasingly acknowledged. Qualitative evidence syntheses (QES) now occupy an important role within the activities of international collaborations, such as the Cochrane Collaboration [1], as part of the guidance production processes of national organisations such as the UK National Institute for Health and Clinical Excellence (NICE) and the US Agency for Healthcare Research and Quality (AHRQ) and as a genuine academic endeavour funded by private, public and charitable funding bodies. Increasingly, QES are viewed as a putative mechanism by which the systematic review “catechism” can be advanced from “what works” to “what happens” [2]. Milestones for the development of QES methodology are well-documented [3]. They include the publication of the first methodology for qualitative synthesis (meta-ethnography) in 1988 [4], the formal recognition of the Cochrane Collaboration’s Qualitative Methods Group in 2006 and publication of the first Cochrane QES in 2013 [5].

Data compiled for the annual Evidence Synthesis of Qualitative Research in Europe (ESQUIRE) workshop in 2015 suggests that between 40 and 70 qualitative syntheses are published each month across a wide range of disciplines with 2-5 methodological references on qualitative synthesis appearing within the same period.

The most cited QES methodological guidance is Chapter 20 of the Cochrane Handbook, authored by co-convenors of the Cochrane Qualitative Methods Group [6]. This was the first document to recognise the potentially important role of qualitative research within the Collaboration. Space constraints limited the searching section to three paragraphs which covered the usefulness of filters, the importance of supplementary searching strategies, an early attempt to highlight the importance of sampling decisions and a cursory sentence on reporting standards. Following receipt of a methodology grant and subsequent methodological summit in Adelaide, the Cochrane Qualitative Methods Group produced supplementary guidance hosted on the Group’s Website. Chapter 3 of this supplementary guidance covered searching for studies [7] mirroring the trial-focused chapter on searching from the Cochrane Handbook. Then, Cochrane policies confined qualitative research to a supporting role within collaboration activities [3] which resulted in potentially useful guidance on supplementary approaches to searching being relegated to an Appendix.

At about this time, the Centre for Reviews and Dissemination was revising its guidance on conducting systematic reviews. For the first time, a chapter on qualitative systematic reviews was included in this seminal guidance. Chapter 6 entitled “Incorporating qualitative evidence in or alongside effectiveness reviews” consisted of 20 pages including just over two pages related to identification of the evidence [8]. Topics covered included a characterisation of search procedures, single paragraphs on sampling approaches and supplementary strategies, respectively, a lengthy discussion of search strategies and filters and a single sentence on reporting standards.

Despite considerable advances in QES methodology, many gaps remain to be addressed. While this is true for all stages of the review process, the place of searching at the beginning of the process renders it a particular priority. Our knowledge of searching for qualitative research is founded primarily on custom and practice. Very few empirical studies exist to inform information retrieval practice. Consequently, we have an imperfect knowledge of the most effective retrieval terms, partial understanding of the respective yield of different sources and, in particular, an incomplete insight of the appropriateness of different sampling methods as they relate to different types of QES.

This methodological review was compiled to support the work of the author and other co-convenors of the Cochrane Qualitative and Implementation Methods Group in writing updated guidance on literature searching for qualitative evidence. In conjunction with a pending major revision of the Group’s chapter in the Cochrane Handbook, the co-convenors of the Group have developed a publishing plan for supplementary guidance, including a chapter on searching. This methodological overview does not duplicate the forthcoming guidance. It documents the evidence base that will inform the guidance, much as a systematic review might inform subsequent clinical guidelines. The aim is to produce a summary of the evidence base for searching for QES that is not constrained by current interpretations of the role of QES within the Cochrane Collaboration. Such a methodological summary may conceivably inform handbooks and other guidance as produced by health technology assessment (HTA) agencies, guideline producers and other review organisations.

Specifically, the author sought to address three methodological questions:

  • What is the current state of knowledge in relation to this aspect of searching practice?

  • How robust is the evidence base for this aspect of searching practice?

  • What are the main gaps and future research priorities for this aspect of searching practice?