Using recognised search terms for support for breast-feeding derived from a previous Cochrane systematic review in the area, we searched for qualitative research on support for breast-feeding using six electronic bibliographic databases: MEDLINE, EMBASE, CINAHL, British Nursing Index, ASSIA and Social Sciences Citation Index. These databases represented the disciplines of medicine, nursing, and social sciences. We evaluated three electronic search strategies: using thesaurus terms; using free-text terms; and using broad-based terms.
Strategy 1: Using thesaurus terms
Indexing systems on electronic literature databases involve systems of controlled keywords (known as thesaurus terms or subject headings) that are used to categorise each record stored. Medline, for instance, uses Medical Subject Headings (MeSH), a highly structured thesaurus. The thesaurus terms that we used varied for each database according to their indexing system. For example, qualitative research is indexed on MEDLINE as "Qualitative Research" or "Nursing Methodology Research", while on CINAHL their subject heading "Qualitative Studies" is complemented by more detailed terms, including "Phenomenological Research" and "Grounded Theory".
Strategy 2: Using free-text terms
This strategy was based on using free-text terms that might specifically identify qualitative research. This strategy searches the titles, abstracts and keywords of records held in the databases. We used over 40 commonly used qualitative research methodology terms, including "ethnograph$", "lived experience$", "narrative analysis", "grounded theory" and "glaser adj2 strauss$". These terms were derived through reference to existing methodology filters for qualitative research  and through our own expertise in information retrieval.
Strategy 3: Using broad-based terms
This strategy was based on using three broad free-text terms, "qualitative", "findings" and "interview$" and the thesaurus term "Interviews". These terms were selected as a result of unpublished research shared with the project team .
The terms used in all three strategies were purposively chosen in order to maximise the comprehensiveness or sensitivity of searching for qualitative methodologies. The thesaurus and free-text strategies included terms that are often associated with quantitative rather than qualitative research, including "questionnaire" and "attitude". It was necessary to include these terms because pilot work suggested that they may also be used by bibliographic databases to classify qualitative studies, even though many qualitative researchers might not choose to use such terms to describe their work.
The "yield" of the searching strategies was assessed as the total number of records identified by each strategy. The total initial yield of all three strategies across the six databases was used as a proxy for the population of qualitative studies in the area of support for breast-feeding. It was not within the scope of our study to determine the "true" population: establishing the "true" population would have required a "gold standard" method of retrieving qualitative evidence, which currently does not exist, and would also have required additional techniques including hand-searching of relevant core journals. Initial analyses indicated that the references were likely to be so widely scattered across journals as to make this a logistically impractical task.
"Relevance" of records identified by the searches was defined by whether the records were relevant to the topic of breast-feeding support and whether they used a recognised qualitative methodology. The judgements of relevance were made by experts in qualitative research and the topic area (RS, TM, JS, BY, MDW and SB). All judgements of relevance were based on the abstracts for the records, or, where these were unavailable (in 23% of cases), full-text articles were obtained. Any ambiguities or difficult cases were settled by consensus.