|This page provides some basic considerations for developing comprehensive search strategies. Both inexperienced and experienced searchers are encouraged to consult with a librarian when conducting a systematic review or other synthesis, particularly when the intent is to publish.|
Quantitative and/or qualitative research questions often utilize the PICO(S) format to identify the key concepts of the question.
|P||Patient, Population or Problem|
For qualitative research, PICO (presented above) or PICo formats may be utilized to identify the key concepts of your question.
|P||Population / types of Participants|
|I||phenomenon of Interest|
When conducting comprehensive database searches, the search terms for each concept should consist of both subject headings (indexed terms) where available, as well as keywords, in order to increase the sensitivity of the search.
It can be extremely helpful if you already know of a few eligible studies for your synthesis. If the eligible studies have been indexed in databases such as MEDLINE and Embase you can review the subject headings that were applied to these studies to harvest a list of relevant search terms. Relevant studies can also be used to identify additional keywords that are relevant for the search.
When conducting a synthesis, the search process is usually very comprehensive as most syntheses strive to find all studies that are relevant to the review topic.
Some syntheses will include the appropriate study design(s) as part of the search strategy (the 'S' in PICOS and PICoS above).
Identifying particular study designs in a database search is not as straight-forward as, for example, limiting to the publication type "Randomized Controlled Trials" in PubMed. This is partly because not all indexed records get assigned the appropriate publication type but also because databases like Ovid MEDLINE/PubMed include non-indexed citations that either have not or will not receive indexing to indicate the study design.
Search filters ("hedges") have been developed and tested for different databases to do just that. These filters can be applied as published or they can be revised to increase the specificity or sensitivity of your search.
Study design filters:
It may also be tempting to exclude publication formats such as letters from your search to reduce the number of results.
If your review topic implies that the research subjects are human, as is often the case with qualitative research, it is advisable to resist the temptation to limit to humans. However, if your search results retrieve a large number of animal studies, you may consider limiting to human studies.
Limiting to humans is not as straight-forward, for example, as using the limit for "Humans" in Ovid MEDLINE. This is partly because indexed records in a database may not have been assigned a subject heading for the appropriate species but also because databases like PubMed/Ovid MEDLINE include non-indexed citations. These records either have not or will not receive indexing to indicate whether the article discusses animals and/or humans.
The recommended method for limiting to human studies in a database is to exclude indexed records that have been assigned a subject heading for animals but that have not been assigned a subject heading for humans. This will remove indexed records that discuss animals only.
For example, this command would read as follows:
|Ovid MEDLINE||*Search# NOT (exp animals/ NOT humans/)|
|Ovid Embase||*Search# NOT ((exp animal/ or nonhuman/) NOT exp human/)|
*Where the Search# represents the search line that you want to limit to human studies.
It is important to note that limiting to human studies is not equivalent to excluding animal studies since the latter would remove studies that discuss both humans and animals. Therefore do not use the command: Search# NOT animals/ (for example).
The most common search errors reported were:
Higgins, J.P.T. & Green, S. (Eds.). (2011). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration. Available from www.handbook.cochrane.org.
Iansavichene, A. E., Sampson, M., McGowan, J., & Ajiferuke, I. S. (2008). Should systematic reviewers search for randomized, controlled trials published as letters?. Annals of Internal Medicine, 148(9), 714-715.