Database limits
Many databases feature an inbuilt set of limiters that may be used to restrict your search. Typically, these can be selected by ticking a box to apply the limits from the database interface. For example, ticking Humans, or ticking English Language.
The use of limits can result in the exclusion of relevant studies, so the use of a search filter in place of a limiter is preferred for systematic reviews.
Search filters
A search filter (also known as a hedge), is a preset combination of terms designed to filter search results to those with specific content. They are in effect a preset search strategy (typically with a combination of keywords and subject headings) which can be incorporated into your search to restrict the results. Filters are often developed by experts and are the most effective way of restricting a search. Commonly used filters restrict to study designs such as RCT's.
Validated filters
A search filter that has been tested and validated against a independent sample ('gold set') of relevant articles and the results published, is known as a 'validated filter'. If you make any changes to a validated filter (to translate it into another database for example), it is no longer a validated filter.
Inbuilt filters
Often you will need to add a search filter line-by-line to your search strategy. Some databases however may have inbuilt filters (in addition to inbuilt limits). For example, the Special Ovid Filters available in the Additional Limits menu on the Ovid platform. The example below shows the Cochrane Library's humans filter available in Medline.
Filter sources
Filters can be categorised into two main groups:
- By study type - methodological
- By topic or subject.
See below for key sources of search filters, including filters for specific study types and designs and/or specific topics:
- ISSG search filters resource (largest range of database search filters)
- McMaster University's HiRU Hedges (Medline, Embase, PsycInfo)
- SIGN filters (Medline, Embase, Cinahl)
- CADTH Search Filters Database (Medline, Embase, PsycInfo, Cinahl, Scopus, Pubmed)
- Ovid expert searches & search filters (Medline, Embase, PsycInfo).
Example - Cochrane search filter
The following example is a filter used to identify randomised trials in Ovid Medline. This and filters for other databases can be found in the Cochrane handbook, Chapter 4. Search filters 3.6.
Box 3.d Cochrane Highly Sensitive Search Strategy for identifying randomized trials in MEDLINE: sensitivity and precision maximising version (2008 revision); Ovid format
1. randomized controlled trial.pt.
2. controlled clinical trial.pt.
3. randomized.ab.
4. placebo.ab.
5. clinical trials as topic.sh.
6. randomly.ab.
7. trial.ti.
8. 1 or 2 or 3 or 4 or 5 or 6 or 7
9. Exp animals/ not humans.sh.
10. 8 not 9
See also Cochrane search filters for Embase.com (see translation into Ovid Embase here) and for Cinahl Plus databases.