Introduction

Systematic reviews seek to identify as many potentially relevant studies as possible that meet the research question for a given review topic. This identification process is best carried out
systematically to minimise bias and searcher oversight. A variety of sources should be searched for any review topic to minimise a range of biases and the particular range of sources will differ between reviews according to the topic.

The following checklist suggests some key sources to search and advises on information tools that may yield further sources of information. The list is not exhaustive and additional resources should be identified, particularly in the areas of non-English language databases. Some sources and methods of searching are well developed and stable, but other areas, such as the proliferating WWW services, are volatile. New tools and searching approaches are being launched daily on the Internet and therefore this section in particular is necessarily prone to rapid change.

It is highly desirable to involve a suitably experienced librarian or information officer in the search process and at the earliest stages of review planning. Librarians are trained to search efficiently and have a wide knowledge of information sources and how to locate information services. They are also essential in helping with document acquisition following the search and may also be able to assist with document and record management issues.

Scoping Searches

The process of conducting a systematic review usually involves, at an early stage, a scoping search. This should typically search for existing reviews and major trials and other studies.
Results from these searches can refocus or focus the review. Following the scoping searches (which are usually only undertaken in a small range of databases, such as MEDLINE, EMBASE and CINAHL), the search question is refined, the full range of possible information sources are identified and the full searches aimed at retrieving the maximum number of relevant studies can begin. This process can involve several iterations. Scoping searches are also used to estimate the size of the literature and by extension the cost of the review.

Full Searches

Once the final research question and search strategy for a review have been decided, the full searches can be carried out to provide as thorough a list as possible of studies, both published and unpublished, which may fit the inclusion criteria and hence be suitable for inclusion in the review.

Information on carrying out literature searches for systematic reviews can be found in the following publications:

NHS Centre for Reviews and Dissemination. Undertaking systematic reviews of research on effectiveness: CRD's guidance for those carrying out or commissioning reviews. 2nd ed. . York: Centre for Reviews and Dissemination, 2001

Higgins JPT, Green S (editors). Cochrane handbook for systematic reviews of interventions version 5.0.0 [updated February 2008]. The Cochrane Collaboration, 2008.

Glanville J. Carrying out the literature search. In: Curran S, Williams CJ. Clinical research in psychiatry: a practical guide. Oxford: Butterworth-Heinemann, 1999, p.57-58.

The Swedish Council on Technology Assessment in Health Care. Literature searching and evidence interpretation for assessing health care practices. (SBU report; 119E). Stockholm: SBU, 1993.
This guide is primarily concerned with identifying, retrieving and interpreting evidence for the assessment of health care interventions, including technologies and other practices.

HTAi Information Resources Vortal
Comprises a large and growing group of health technology assessment (HTA) related web site links, including information on literature searching.

Royle P, Waugh N. Literature searching for clinical and cost-effectiveness studies used in health technology assessment reports carried out for the National Institute for Clinical Excellence appraisal system. Health Technol Assess. 2003;7(34).

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