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Research Guides@Tufts

Systematic Reviews

An introduction to the requirements, search strategies and resources needed to conduct the literature review portion of a Systematic Review

Popular keyword databases

Boolean logic

When searching any database - but especially keyword databases - it is important to keep in mind that the computer is relatively stupid. It is only searching for the string of letters found in the words you type into the search. In order to ensure your keywords are combined effectively, most keyword databases allow you to use Boolean logic. The three most common operators (they tell the database how to operate) are AND, OR, NOT. The venn digaram below illustrates how search results are generated using each operator.

Venn diagram illustrating Boolean Logic for terms AND, OR, NOT

Some helpful tips:

  • Think of AND as a plus sign - both terms must be present in the result
  • OR is typically used with synonyms - you don't care which term appears in the result as long as at least one is present
  • AND will narrow your search results
  • OR will expand your search results

Order of operations

Computers are programmed based on mathematics. It therefore makes sense that you must tell it symbolically how to arrange your search terms to create a powerful and strategic search. Just as in math equations, parenthesis are important indicators of which operation to carry out first or combine.

((cat OR dog) AND (spay OR neuter OR fix)) NOT pet

The above search tells the database that I am looking for information about spaying and neutering of cats or dogs, but I don't want the word "pet" to appear out of all the combined results. Notice that my synonyms for neutering are strung together with OR.

Nested parenthesis can be tricky, but they can help you create more specific searches.


Another common search operator is the wildcard or truncation symbol. Different databases use different symbols (*,?, etc.), but when placed at the end of a root, they allow all ending variations to be searched. For example:

leav* would find leave, leaving, leaves, leaver, leavened, etc.

Be careful not to use too short of a root word, or you might retrieve millions of irrelevant articles with terms you never intended to use.
Think about all the words that could be formed if you entered bat*


Quotation marks or NEAR can indicate to a databse that your want the terms to appear as a phrase or within a few words of each other. Example:

"childhood obesity"
obesity NEAR child*


When you do not have access to a controlled vocabulary, you are at the mercy of human language variants. One idea can be described in a variety of different ways, using multiple terms, and the terms used to describe the topic may have changed throughout time.

  • It is up to the researcher to brainstorm all the synonyms and historic terms for a topic and combine them before executing a search.
  • It is also worth noting that different countries may spell terms differently; such as the British variations of oestrogen, colour, etc.

Some form of organizational chart for developing search terms can be very beneficial when brainstorming synonyms. In the following document, separate ideas receive their own column, while terms are written in rows below them. all terms within a column should be synonyms or spelling variants and combined with OR in searches. Different columns are typically combined with AND.

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