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PubMed Tutorial from The Levy Library: Quotation Marks

Using Quotation Marks (Infrequently)

Using quotation marks allows you to run an exact phrase search in PubMed, meaning the words included in your search result will appear in the exact order in which they appear in the search.  

>>>In most cases, using quotation marks will unnecessarily narrow your search results.

Try this: Search "sperm motility" in the search box, surrounded by quotation marks. Note how many results this search yielded and check the Search Details box. Note that your term did not map to the Sperm Motility MeSH term, since using quotations turns OFF automatic term mapping

Now run a search for sperm motility without quotations.  How many more results did this search yield? Check out the search details box.

The search with quotes will only have results for the exact phrase sperm motility.  The search without quotes will include citations that include phrases such as motility of sperm, sperm count and motility, and others in which both the words sperm and motility appear but not necessarily in that order.

>>>However in some cases, quotes can be used effectively.

Try this: Search for near death experience. Note how many results this search yielded and check the Search Details box. Which search term(s) mapped to which MeSH term(s)?

You should have retrieved over 600,000 results, and most of these are not on target. 

Try this: Now search "near death experience" surrounded by quotation marks. You should retrieve about 130 relevant results.

It is usually best to use quotation marks to find a phrase only when PubMed is processing your search differently than you would like. Also, note that if you use quotes for one concept in your search, you must use an AND to combine it with other concepts (whereas without quotation marks the AND is implied in the absence of another boolean operator [OR, NOT]).