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Systematic searching: 9. Limiting the search

Limiting the search

Many databases provide different filter tools that allow you to limit your search results. The most common filter tools for limiting search results are publication language, year of publication, and peer-review. Other filters may include, depending on the database, e.g. age group or publication type. 

Keep in mind, however, that limiting your search results with filter tools should only be used with caution when undertaking a search for a systematic review since they may remove relevant documents from your search results. Filter tools may also eliminate the most recent documents from your search results if they have not yet been fully indexed into the database. The reporting and reproducibility of the search strategy also suffers from the use of filter tools, as the database algorithms are not transparent, i.e. we do not know for certain exactly how the search engine functions.  

NOTE. In a systematic review, all search limiters must be both defined before the search is undertaken as well as described in the protocol. You also need to be consistent when applying search limiters. 

Limits

  • Research method. There are pre-tested search filters (also called hedges) for different types of methods, e.g. qualitative research. Search filter sites are listed in Section 3.
  • Publication type. Many databases (including PubMed and CINAHL) allow searches to be undertaken by publication or document type, e.g. review, research, randomized controlled trial, editorial or letter.  Filters can also often be used in reverse (NOT/Exclude) by excluding literature reviews, for example. Pre-tested search filters mentioned on this page may be a more comprehensive and reproducible way of limiting the search.  
  • LanguageUse with care. It would be ideal to include all relevant research no matter what language it was originally published in. This way language bias can be avoided. Unfortunately often limited resources do not make this possible. Read more about the language bias phenomenon in the article by Dawid Pieper and Livia Puljak (2021). If you do use language limits it's often recommended to do this at full text screening phase. This way you can decide, depending on the number of results, to examine relevant abstracts of foreign language papers as the abstracts are likely to be in English and may prove of interest even if you cannot include them completely.
  • Date limit.  Use with care. In many cases it is best to avoid date limits if at all possible when doing a systematic review as you are aiming to be as comprehensive as possible. Sometimes date limits should be used, e.g. when updating a previously conducted search. Read more on Updating a search page of this guide.
  • Full text. Do not use. Systematic reviewers should not select this option as it will introduce bias into their research. The availability of the full text will be examined at the full-text stage of the screening process. In this case excluded references will be documented in the Prisma chart.  If systematic reviewers locate articles which appear relevant at the abstract level but which they cannot access through full text subscriptions by their institutions they should try to source them through inter-library loan service.