Searches can often produce large numbers of results. This may be an appropriate number for a systematic review, where you need to ensure your search is very comprehensive. However, if your search retrieves several irrelevant results, techniques can be used make your search more effective.
Ways to restrict and widen your search
If your search only finds a few references, or most of the references are irrelevant to your research topic, consider the following:
- Check your spelling: databases will not usually auto-correct, so they will only find what you type.
- Use a broader search question.
- Do you need to search more databases?
- Could you add more search terms? Look for variations in spelling and alternative words.
- Discuss your topic with your supervisor.
If you have too many results, you may have to focus your search and make it more specific. Ask yourself:
- Have you used Boolean operators correctly? For example, have you used AND when you should have used OR?
- Could you limit it by date range?
- Could you limit the search by language eg English only? Using non-English papers helps to minimise bias, but only if you can translate them accurately.
Use search limits
Each database offers a different range of limits. Many databases allow you to limit your searches by publication year, language and publication type. Some may even allow you to restrict your search to specific types of query.
Limits should be applied at the end of your search after you have retrieved all the results relevant to your topic. Apply limits one at a time so you can see what effect they have on your results.
Using too many limits may make your search too narrow. In this case, you could use search filters instead.
For further information, see the "Using limits" section of the Medline workbook (PDF).
Advice on applying limits in CINAHL can be found on page 8 of the Advanced CINAHL workbook (PDF).
How search filters can improve your search
Search filters are pre-tested strategies that identify specific literature. Filters can be used to restrict your search in a number of ways, for example to limit your results to RCTs, observational studies, or economic evaluations. Example search filters can be found on the Scottish Intercollegiate Guidelines Network (SIGN) website or the InterTASC website. Search filters are added on to the end of search strategies using the Boolean operator "AND". Note that search filters are database specific so will not usually work in other databases.
OvidSP users with a personal account can access two expert filters designed by the Cochrane Collaboration for the purpose of restricting search results to randomised controlled trials.
Search by hand to catch anything you have missed
Handsearching involves selecting key journals in your subject area and searching each one individually, by hand or electronically, with specific criteria in mind. It can also be used to search for sections/chapters in books.
It helps you to pick up material that might otherwise be missed. No database search strategy is perfect, as there may be items that are not indexed or are improperly indexed, and not all materials you need may be indexed by databases. You might also have made some mistakes in the search.