On the previous page, we learned how to identify and curate keywords and subject headings related to your research topic. On this page, we'll cover how to combine those terms into comprehensive database search strategies using Boolean Operators.
Boolean Operators help control how search terms are combined and how results are filtered. They tell the database what kind of relationship you want between different concepts. The three most common Boolean Operators used in database searching are AND, OR, NOT.
The AND operator narrows your search by connecting two or more different concepts together.
When you use AND, you are telling the database that the results must include all of the terms you specify, not just one of them. Let's apply this operator to PubMed search strategies for our example research question: Does TENS (transcutaneous electrical nerve stimulation) reduce pain from primary dysmenorrhea?
In the image above, we ran a search in PubMed using keywords for two of the primary concepts in our research question and connected the terms using AND. This search is telling PubMed to find articles that contain both of these terms.
Let's run another search that includes our third concept, transcutaneous electrical nerve stimulation.
What happened in our new search?
This new search shows how using AND narrows your search results. We added a keyword for the third concept in our research question ("transcutaneous electrical nerve stimulation") using the AND operator.
This additional term is telling PubMed that it needs to show results for articles that contain all three of these terms. In this example, the effect was fairly drastic: dropping the results down from 206 to 13.
The OR operator broadens your search by connecting related terms or synonyms.
When you use OR, you are telling the database that the results can include any of the terms you input, not just a specific one. Let's review the image examples below.
Searching for "transcutaneous electrical nerve stimulation" alone yields 2,749 results. What happens when we also search for our other synonyms and related subject headings/MeSH terms?
What happened in our new search?
This second search shows how using OR broadens your search results and highlights the importance of identifying related keywords, synonyms, or subject headings for each concept in your research question.
This search strategy is telling PubMed that it needs to show results for articles that contain any of these specified terms or subject headings, hence the jump from 2,749 results to 33,015.
Remember, OR gets you MORE!
The NOT operator narrows your search by excluding specific terms.
When you use NOT, you are telling the database to leave out any articles that contain the words or phrases you specify. Other common uses of the NOT operator include using it to exclude animal studies or certain publication types from results.
In the previous tab, we saw our results jump to 33,015 by using the OR operator in the search: ("transcutaneous electrical nerve stimulation" OR TENS OR "Transcutaneous Electric Nerve Stimulation"[Mesh]). Let's say we wanted that same search to exclude animal studies.
While 25,000 results is still a lot to sift through, we can see that adding the NOT operator alone eliminated almost 8000 results! This is without doing anything else to narrow or filter the search.
A strong, comprehensive search usually uses both AND and OR operators to connect your ideas and synonyms.
When you group related terms together using parentheses ( ), you are creating "nests" - a technique called nesting terms.
Nesting helps the database understand exactly how your search terms are related, and ensures that you retrieve the most relevant articles.
If we were use AND and OR operators to combine keywords, phrases, or subject headings from each of our three research question concepts to create a comprehensive search strategy in PubMed, it might look something like this:
What is this search strategy telling PubMed to do?
In other words: PubMed is being instructed to find articles that talk about TENS, pain management, and dysmenorrhea all together - using any of the different terms we provided for each concept.
Two common challenges that arise in database research are finding either too many or too few results. Sometimes, this is unavoidable depending on your research topic.
Subjects that have been extensively studied tend to produce large numbers of results, while topics such as novel treatments or rare conditions, may only yield a few key studies. However, there are several strategies you can employ to further refine your database searches to narrow or expand your results as needed.
| Problem | How to Fix It |
| Too Many Results |
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| Too Few Results |
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