There are currently over 34 million citations in PubMed. If you create the right search with the best search terms and use the search tools in PubMed properly, you will likely return a suitable portion of the literature. Unfortunately, people often find that their searches are yielding either way too many or far too few results. Here are some tips and information regarding how you can broaden or narrow PubMed searches.
PubMed contains a variety of tools and methods to assist with narrowing (focusing) a search. Listed below are some of the ways you can narrow the amount of search results you are getting in PubMed.
1) Use more specific search terms (for example, instead of searching "headache" search for a specific type of headache such as "migraine")
2) Add additional search terms using "AND" (this will only yield results which include, in the abstract or citation, the search terms you used); to aid in discovering additional search terms, consider using the Advanced Search tool in PubMed
3) Set filters and create custom filters through a MyNCBI account
4) Use the "Clinical Queries" search tool to limit results to evidence based literature
5) In Advanced Search, use History to combine previous searches
6) Set filters (see instructions below for setting filters)
Despite the huge number of citations in PubMed (now over 34 million), sometimes searches can yield far fewer results than we would like to find on a topic. Sometimes this is because there are few, if any, articles written on a topic (especially when the topic is a brand new or cutting edge topic). But sometimes there are plenty of articles on the topic, and all that we need to do is take steps to broaden our search to retrieve additional topical articles. The following steps can help you "throw a wider net" and broaden the amount of articles yielded by your search.
1) Add synonyms (place in parenthesis) with OR between the terms (for example, if searching using just sickle cell anemia is bringing too few results, try building a search which includes synonyms for the disease constructed in a similar manner as in the following example: (sickle cell anemia OR sickle cell disease OR hemoglobin s disease OR sickle cell disorder)
2) Remove filters (try removing them one or two at a time; you will not necessarily want or need to remove all of them)
3) Remove terms which have an AND between them (by default, an AND is placed between terms in the PubMed search box; so if we do a search using the terms pregnancy women, and find we are getting too few results, we can rerun the search using just the term pregnancy since we can deduce that including the word women is unnecessary)
4) See "Similar Articles" (in the abstract view of a citation in PubMed, you will find similar articles near the bottom of the article. You can navigate directly to this section by clicking on "Similar Articles" on the right side of the page. To see even more related citations, click on the "Show more similar articles" link.