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3. Searching electronic sources

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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:

logo VDIC  More than 10.000 e-journals and 8700 Ebooks (IP recognition)

logo CDLH Access to databases, journals and eBooks via CEBAM DLH (login required)

3.1 Sources of biomedical literature

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

Core databases

  1. MEDLINE contains records from 5600 journals (39 languages) in the of biomedical field, from 1946 onwards (Access for KCE | Free access through PubMed).
  2. EMBASE: Records from 7600 journals (70 countries, 2000 not covered by Medline) in biomedical field, from 1974 onwards (Access for KCE).
  3. CENTRAL - The Cochrane Controlled Trials Register, part of the Cochrane Library: Records of randomised controlled trials and controlled clinical trials in healthcare identified through the work of the Cochrane Collaboration including large numbers of records from MEDLINE and EMBASE as well as much material not covered by these databases (Dickersin, 2002). (Access for KCE through CDLH | Free access to abstracts)

Databases for systematic reviews

  1. CRD Database of reviews of effectiveness (DARE) contains structured abstracts, including critical appraisal, of systematic reviews identified by regular searching of bibliographic databases, and handsearching of key journals. [the update of CRD DARE has ceased March 2015]
  2. Cochrane Database of Systematic Reviews (CDSR, part of the Cochrane Library) lists the results of systematic reviews (full text) conducted by Cochrane groups, but also ongoing projects (Access for KCE through CDLH | Free access to abstracts)
  3. Special queries exist for Medline or Embase to limit the identified records to articles identified as Systematic reviews. See appendix.

Databases for HTA reports

  1. The INAHTA HTA database is a bibliographical database of published HTA reports; it also lists ongoing HTA projects. Members of INAHTA are regularly invited to update their information on the HTA database, informaiton from the main HTA producers is also collected by the database maintainer. Access is free, records of the HTA database are also searchable via the Cochrane Library.
  2. HTA reports can also be found at individual agencies’ sites, the HTAi vortal lists HTA organisations, it also provides a custom Web search engine that limits the Google results to pages published on the website of HTA organisations listed on the HTAi vortal.

Databases for specific topics

  • Nursing: CINAHL (Cumulative Index to Nursing and Allied Health Literature), British Nursing Index (BNI) (Access for KCE through CDLH)
  • Physiotherapy: PEDro (contains records of RCTs, systematic reviews and evidence-based clinical practice guidelines in physiotherapy, from 1929 onwards; most trials in the database have been rated for quality to quickly discriminate between trials that are likely to be valid and interpretable and those that are not; free access)
  • Psychology and Psychiatry: PsycInfo (Access for KCE)
  • More bibliographic databases are listed on the KCE library catalogue (e.g. CAM, ageing, ...)

3.2 Sources of economic literature

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

Core database

  • NHS Economic Evaluation Database (NHS EED) contains over 7000 abstracts of quality assessed economic evaluations. The database aims to assist decision-makers by systematically identifying and describing economic evaluations, appraising their quality and highlighting their relative strengths and weaknesses. [the update of CRD NHS EED has ceased March 2015]
  • Some of the search filters for Medline or Embase limit the records to articles related to Costs, Economic evaluations, Economics

Complementary databases

  • EconLit:database of economics publications including peer-reviewed journal articles, working papers from leading universities, PhD dissertations, books, collective volume articles, conference proceedings, and book reviews  (Access for KCE)

3.3 Sources of clinical practice guidelines

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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:

3.4 Sources of ongoing clinical trials

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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:

3.5. Sources of grey literature

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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

3.6 Building a search strategy

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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.

3.6.1 Search tools

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

Boolean and proximity operators

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 & wildcards

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.

3.6.2 Search limits

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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:

  • Focus / Major Heading: limits to the articles that have been indexed with the term as Major Heading. This helps to reduce the amount of results (up to 40%) while keeping a good pertinence thanks to the human indexation of the full article (in case of Medline and Embase).
  • Subheading: these are also added to the description of an article by the indexers, but should be used with more precaution (can render the search strategy too restrictive).

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.

3.6.3. Search filters

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

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.

3.7 Documenting a search strategy

Author(s): 
Joan.Vlayen
Author(s): 
Patrice.Chalon

The search strategy for electronic databases should be described in sufficient detail to allow that

  • the process could be replicated
  • an explanation could be provided regarding any study not included in the final report (identified by electronic sources search or not)

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).

AttachmentSize
process_04_template_-_search_strategy_1.doc38 KB
process_04_template_-_search_strategy_1.odt10.76 KB