Refining your search

Searching databases is an iterative process. As you review your results, you may decide to refine your search strategy to include, exclude or amend some search terms, limits and/or filters. This is a normal part of the literature searching process.

Here we offer some advice on strategies to use to address the most common search problems:

If you have too many hits

  • Be specific: Use search terms that are as precise as possible.
  • Boolean operators: Use the powerful AND or NOT operators to refine your search. Use NOT with caution!
  • Limit: Reduce the parameters of your search by selecting publication year, language, publication type, etc. There is a full range of limits, but use some with caution, e.g. restricting your search to articles in English is an arbitrary measure, probably excluding excellent research.
  • Focus: Some databases have a “major heading” option for subject heading terms, restricting the search to articles with your term as a main subject. Again, use this with caution as you may miss some excellent articles.
  • Search filters/methodology filters (see the “Limits and Filters” section earlier): Apply ‘ready-made’ search filters to find the right data. Some databases have them as Limits, or you can use methodological filters for systematic reviews, meta-analyses, etc.
  • Subheadings: Some databases have subheadings – shortcuts to popular facets of research, varying by subject heading. Be cautious with subheadings – they are not always robust, and you may miss relevant articles.

If you have many irrelevant hits

  • Subject Headings: You will retrieve a higher proportion of relevant articles if you search with subject headings rather than with free-text alternatives.
  • Permuted Index (Ovid interface – in the Tools menu): Search for all subject headings containing your word, you will be able to select the most appropriate subject headings from a list.
  • Thesaurus Display: Use the subject tree/index to find more precise subject headings.
  • Limit: See above.
  • Boolean operator NOT: A search on ‘pregnancy’ will retrieve a fair number of articles also about childbirth. Searching for ‘pregnancy NOT childbirth’ will exclude the articles including childbirth. Beware! You may exclude many useful articles.

If you have too few hits

  • Check your spelling: It may seem obvious, but incorrect spelling, particularly in free-text searching, will reduce the number of results. It’s easy to spell cattle with three “t”s, for example! Beware of alternative spellings during textword / keyword searches (e.g. behaviour / behavior; immunisation / immunization).
  • Explode: Use the explode option to include a broad subject heading and all the narrower terms branching off from it (this is easier to visualise if you look at the Tree Display).
  • Avoid subheadings: Select ‘All Subheadings’ when you are presented with the option, because the subheading system is not entirely reliable.
  • Synonyms: Most database content is international, so if your search terms do not map to any appropriate subject headings, think of North American or other equivalents.
  • Lateral searching: Look at the subject headings tagged onto a relevant article. Use those terms to expand the scope of your search.
  • Free-text searching: An alternative for terms that are too new or are not sufficiently widely used to be subject headings.
  • Related terms: Use the truncation symbol (the symbol can vary between databases, but the most common one is *) in your textword / keyword search to retrieve words with a common root. ‘Tubercul*’ will bring up tuberculosis, tuberculin, tubercule, etc.
  • Search other databases too: No database is complete. In addition to CAB Abstracts and Medline/PubMed, try the Biosis Citation Index, Web of Science, etc.
  • Avoid limits: Especially ones that don’t influence the quality or relevance of search results (e.g. abstract only).