3.6 Building a search strategy
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
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
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
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:
- PubMed at the Clinical Queries screen
- InterTASC: http://www.york.ac.uk/inst/crd/intertasc/index.htm
- SIGN website: http://www.sign.ac.uk/methodology/filters.html
- HiRU: http://hiru.mcmaster.ca/hiru/
- OVID or Embase.com
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.