Another one of the main differences between a systematic review and the more traditional, narrative, non-systematic review is the exhaustive literature search that must identify all primary studies exploring the review question in order to produce an objective answer (Sambunjak & Franic 2012).The search strategy should be as
comprehensive as possible in order to minimise bias (Victor 2008) that may arise by inclusion of only particular types of evidence, such as may occur due to the presence of pre-formed opinions (Centre for Reviews and Dissemination 2008). This step of the systematic review requiring location and retrieval of all relevant literature is
challenging yet vital to the success of a systematic review, as the literature obtained provides the information on which the evidence, conclusions and recommendations are established (Smith et al. 2011). In this regard, this process of formulating the search strategy may benefit from the assistance of an experienced librarian (Mead & Richards 1995). Finally, it is also essential from the outset of the review that the reviewer bear in mind the scope, the comprehensiveness and the time available to conduct the search (Smith et al. 2011).
Citation searching is an important procedure in addition to database and hand searching (Greenhalgh & Peacock 2005). Relevant studies may be found by manually searching reference lists of identified relevant studies, known as backward citation searching, or by finding the articles that have cited the included studies, known as forward citation searching (Bates 1989). Citation indexes, such as Science Citation Index or Science Citation Index Expanded, can assist this process of identifying additional studies for inclusion in a review (Higgins & Green 2011).
Various forms of evidence can be included which encompass published and unpublished research, as well as ‘grey’ literature such as, government documents, theses, conference papers, and research studies in progress (Victor 2008). As forms of
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‘grey’ literature may not be peer-reviewed, some reviewers do not include these forms of evidence (White & Schmidt 2005). Despite this, others suggest that including ‘grey literature’ mayadd richness to the review and minimise bias (Benzies et al. 2006). In addition, it is also suggested to minimise avoid over-representation of studies with statistically significant findings which may produce inaccurate estimates of effect sizes (McAuley et al. 2000, Conn et al. 2003).
Methods for identifying applicable research studies include searching databases and research registers, citation tracking, and contacting academics and specialists in the field of interest (Victor 2008). A high-quality electronic search is vital to ensure the accuracy and completeness of research article retrieval for use in a systematic
reviewing (Volpato et al. 2014). Nonetheless, the process of devising search terms for use in electronic research databases can often be quite complex and require careful testing to improve the rate of retrieval (White & Schmidt 2005, Victor 2008). The balance between sensitivity, the power to detect all relevant research on a particular topic of interest, and specificity, the ability to eliminate irrelevant studies (Victor 2008), is important to ensure that all relevant research is included in the review (White & Schmidt 2005). In addition to the creation of database-specific search terms, the quality of retrieved information also depends on the researchers’ knowledge of each database’s controlled retrieval language and searching tools in order that to search results can be adjusted to meet the needs for specificity or exhaustiveness in information retrieval (Volpato et al. 2014). As differences have been claimed in the data retrieval between the various databases (Brazier & Begley 1996, Michaleff et al. 2011, Elsevier 2015), at least seven electronic databases should be searched, for example, CINAHL, MEDLINE, EMBASE, Cochrane Library (Thomas et al. 2004). CINAHL® Plus with Full Text is a privately owned database (Lawrence 2007) delivering full text for more than 760 journals and indexing for more than 5,000 journals dating back to the year 1937 from the areas of nursing, biomedicine, health sciences librarianship, alternative/complementary medicine, consumer health and seventeen allied health disciplines (EBSCOhost 2016). In total, the database contains more than four million records and also offers access to health care books, nursing dissertations, certain conference proceedings, standards of practice, educational software, audio-visuals and
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book sections (EBSCOhost 2016). Lawrence (2007) describes CINAHL as a powerful and valuable indexing tool particularly in the field of nursing and nursing research. Focusing more on biomedicine and, to a lesser extent, life sciences, MEDLINE is a bibliographic database produced by the U.S. National Library of Medicine and forms the primary component of PubMed (Young & Duffull 2011, U.S. National Library of Medicine 2015). Generally dating from the year 1946 to the present, MEDLINE offers over 22 million journal article references from more than 5,600 worldwide journals (U.S. National Library of Medicine 2015). Comprising of over 30% of journal titles not covered by MEDLINE, EMBASE is another large database that references more than 8,500 journal titles which span sixty subject fields (Elsevier 2015). EMBASE has a broad biomedical scope and has a particular focus on the coverage of pharmacology, pharmaceutical science and clinical research that represent over 70% of EMBASE content (Elsevier 2015). EMBASE also includes basic biomedical science, veterinary science and allied health topics (Elsevier 2015). As mentioned, the Cochrane Library is the main output of the Cochrane Collaboration and comprises of six databases that contain different types of independent evidence to inform healthcare decision-making, as well as a seventh database that provides information about Cochrane groups (Cochrane Library 2015). The Cochrane Library contains more than five thousand full-text reviews on the effectiveness of healthcare interventions and also includes critical assessments of systematic reviews, of which are updated regularly (Cochrane Library 2015). The Cochrane Database of Systematic Reviews (CDSR) is a prominent resource for systematic reviews in health care and includes Cochrane reviews and protocols prepared by Cochrane Review Groups, all of which are peer-reviewed.
Considering the differences in the focuses across the numerous electronic data
resources alongside the subsequent variation in contents, the databases employed in a data search should be carefully selected in relation of the review’s topic of interest and ideally not restricted to language-specific databases (Sambunjak & Franic 2012). In this regard, designing a search strategy that searches only one electronic database may reduce the likelihood of retrieving all the relevant data and, in turn, increase the risk of bias. In sum, it is imperative that the researcher acknowledge that the quality of any of
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the conclusions made following a review are only as good as the quality of the processes employed to search and retrieve the evidence (Bettany-Saltikov 2010b).