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Systematic reviews: Understanding search strategies

A quick guide to introduce you to systematic reviews

Search strategies

Search strategies for systematic literature searches can often appear complex and difficult to understand. This section of the guide runs through some search techniques in practice, using a search strategy from a Cochrane systematic review as an example.

Searching specific fields

Sometimes searchers often choose to focus their search, limiting their searches to specific database fields to increase the relevancy of search results. Many systematic searches target the title and abstract fields of a database record. 

You can identify when a search has been run on specific fields as these search lines often have codes before or after the search term (i.e. ti,ab OR TIAB). You can decifer which fields are being searched in a strategy by looking at the field code on each search line alongside the help guide for the database that the authors have searched. This should provide a key to all the database fields and their corresponding field codes.

Filters and search techniques

The Cochrane search strategy on the right uses a validated publication type filter (lines 1-9) to very sensitively limit results to randomised controlled trials. In these lines, you will see that the field code .pt has been used to limit keyword searches to 'publication type'. The authors have also searched for terms commonly used within randomised controlled trials to pick up studies that don't have any information in the publication type field.

In line 10 of the Cochrane search, you'll see that the authors have used an animal studies filter to remove all studies that do not relate to humans. This search filter uses the Boolean NOT operator to remove all of the animal studies that may be retrieved.

To ensure that any relevant studies are not missed by just searching using keywords, the authors have searched for a relevant MeSH (subject heading) term relating to the search for breast cancer in line 12. This is using the database thesurus to make sure the search strategy reflects the possible language used within the database.  The exp before the Breast Neoplasm means that the database will also retreive articles using more specific terms relating to breast neoplasms.

In lines 13-16, some advanced search techniques have been used to ensure all relevant studies are retrieved. Proximity searches can be identified by the adj6 (which means in this case one term will be looked for within six words of another term in either direction). The dollar symbol $ is an example of truncation wherby the author wishes to ensure both singular and plural word variations are retrieved in the search.

Understanding database fields

Understanding database fields can help you interpret the complex search strategies used for systematic reviews. A list of database fields for Ovid Medline is available which details all of the fields available for Medline on this platform. The default search field in Ovid advanced search is mp (multi-purpose) which searches across a range of different fields, including title and abstract.

The image below shows an article record in Ovid Medline, so you can see where the different fields appear in an article record: 

Example of a search strategy in Ovid Medline

The ‘adj6’ operator indicates within six words;

‘$’ indicates truncation;

.mp. indicates a search of title, original title, abstract, name of substance word and subject heading word.

Higgins JPT, Green S (editors). strategiesBox 6.4.fCochrane Handbook for Systematic Reviews of Interventions. Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available from www.handbook.cochrane.org

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