The decision on which source to use depends on the research question. The three electronic bibliographic databases generally considered being the richest sources of primary studies - MEDLINE, EMBASE, and CENTRAL - are essential in any literature review for the KCE. However, many other electronic bibliographic databases exist.
Systematic reviews can be found in the Cochrane Database for Systematic Reviews, in DARE or in Medline. Search strategies have been developed to enhance the identification of these types of publications (Kastner, 2009; Montori, 2005).
HTA reports can be found in the HTA database of INAHTA or at individual agencies’ sites (see HTAi vortal under "HTA Agencies and Networks").
Specifically for drugs and technology reviews, data from the US Federal Drug Administration (FDA) or EMA can be helpful.
Providing an exhaustive list of all potential sources is not possible here. The KCE library catalogue provides a list of such sources.
Access to electronic resources happens through the following digital libraries:
More than 10.000 e-journals and 8700 Ebooks (IP recognition)
Access to databases, journals and eBooks via CEBAM DLH (login required)
Core database
Complementary databases
Often, specific guidelines can only be retrieved through local websites of scientific associations or government agencies. It is therefore recommended to combine a Medline search (with specific filters for guidelines) with a search of the following:
Ongoing trials may have limited use as a means of identifying studies relevant to systematic reviews, but may be important so that when a review is later updated, these studies can be assessed for possible inclusion. Several initiatives have been taken recently to register ongoing trials:
More and more electronic sources describe "grey literature" (results of scientific research not published in scientific journals; e.g. reports, working papers, thesis, conference papers, ...)
Institutional repositories
For each database, search terms defined in the preparation phase will be mapped to the Thesaurus terms of the database (when available). Mapping can be achieved using the built-in functionality of the search interface, or manually by looking at the indexation of previously identified pertinent articles. Attention will need to be paid to the explosion tool (sometimes selected by default linke in PubMed, sometimes not like in OVID Medline).
The most important synonyms of the Thesaurus terms identified for each facet will also be added to the search strategy as text word. Advanced functionalities of the search interfaces will be used (see below: truncation, wildcard, proximity operators).
The terms within a specific facet will be combined with the Boolean operator ‘OR’ in order to group all articles dealing with this facet. For some concepts, special queries (also called search filters) have been developed (see below). The resulting groups of articles will then be combined using the Boolean operator ‘AND’.
It is recommended to validate each search strategy by a second reviewer.
In the context of database searching, Boolean logic refers to the logical relationships among search terms. Classical Boolean operators are ‘AND’, ‘OR’ and ‘NOT’, which can be used in most databases. Importantly, in some databases, such as PubMed, these Booleans need to be entered in uppercase letters. Other operators, the so-called proximity operators, are ‘NEAR’, ‘NEXT’ and ‘ADJ’. A more detailed overview of Boolean and proximity operators is provided in Appendix.
Truncation can be used when all terms that begin with a given text string are to be found. Different databases use different characters for truncation with different functionalities. For example, in PubMed, OVID and EMBASE ‘unlimited’ truncation is represented by the asterix ‘*’, but OVID Medline also uses ‘$’.
In OVID Medline the ‘optional’ wildcard character ‘?’ can be used within or at the end of a search term to substitute for 1 or 0 characters. In contrast, in EMBASE a question mark indicates exactly one character.
A more detailed overview is provided in appendix.
When the amount of resulting hits is too high to be managed within the available timeframe / resources, search limits may be applied.
First, tools related to the Thesaurus should be considered:
Several search interfaces provide search limits that can also be applied to narrow the search. Classical examples are date and language limits, but some databases also provide limits according to age, gender, publication type etc. Before applying search limits, the risk of a too specific (i.e. narrow) search should be considered.
In systematic reviews, if time and resources allow, specificity is often sacrificed in favour of sensitivity, to maximize the yield of relevant articles. Therefore, it is not unusual to retrieve large numbers (possibly thousands) of bibliographic references for consideration for inclusion in an extensive systematic review. This means that reviewers may have to spend a lot of time scanning references to identify perhaps a limited number of relevant studies.
Search filters are available to focus the search according to the type of study that is sought, for example to focus on randomized controlled trials, diagnostic accuracy studies, prognostic studies or systematic reviews (see example in appendix). Specific search filters also exist for well-circumscribed clinical problems/populations, e.g. child health (Boluyt, 2008), palliative care (Sladek, 2007), or nephrology (Garg, 2009).
Sources of filters include:
During the selection of an appropriate search filter, aspects of testing and validation should play an important role. Specific appraisal tools are available to evaluate the methodological quality of search filters (Bak, 2009; Glanville, 2009).
For diagnostic studies, it is recommended not to use a search filter.
The search strategy for electronic databases should be described in sufficient detail to allow that
The template required by KCE to describe a search strategy is provided in attachment.
All identified references must be exported, preferably in a text file to be imported in a Reference Management Software (see appendix for technical description).
Attachment | Size |
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process_04_template_-_search_strategy_1.doc | 38 KB |
process_04_template_-_search_strategy_1.odt | 10.76 KB |