Searches for systematic reviews aim to be as extensive as possible to capture all available studies relevant to the research question. Thus, it is important to strike a balance between striving for comprehensiveness (sensitivity) and maintaining relevance (precision) when developing a search strategy. Precise or focused searches retrieve less irrelevant results but have a higher risk of missing targeted literature. Sensitive or broad searches retrieve most of the targeted literature, but also return a higher proportion of irrelevant results. Increasing the comprehensiveness or sensitivity of a search will reduce its precision and retrieval of relevant studies.
The sensitivity and precision of a search strategy are determined by the search concepts and terms (see Step 2). The more concepts that are added to the strategy with AND, the more precise and fewer results. The more search terms or synonyms that are added to the strategy with OR, the more sensitive and more results.
Boolean operators (AND, OR) are used to combine concepts and search terms in a database. It focuses the search, particularly when the concept contains multiple search terms or synonyms. It also connects various pieces of information or concepts together.
"diabetes mellitus" AND "telemedicine"
"telemedicine" OR "mobile health" OR "mhealth" or "telehealth"
Putting it together
Databases follow commands you specific and return results based on those commands. Keep in mind, Boolean operators assign logical order and commands to which terms/concepts get searched first. PubMed executes search commands from left to right and adds parenthesis to each step (see Search #1 and #2). If no boolean operators are specified, PubMed assumes each term is combined with AND (i.e. implicit AND) (see Search #4 below). If you are using a combination of AND or OR operators in a search, enclose the terms that are OR-ed together in parentheses
"diabetes" AND "mobile health" OR "telemedicine"
Your results will include articles that relate to 1) diabetes and mobile health, or 2) telemedicine (see Search #1)
"diabetes" AND ("mobile health" OR "telemedicine")
Your results will include articles that relate to 1) diabetes and mobile health, or 2) diabetes and telemedicine (see Search #3).
Tip: To see how PubMed interpreted your search, click "Advanced" under the search bar, and in your search history, click on the arrow icon in the "Details" column.
From the example earlier:
("diabetes OR "type 1 diabetes" OR "type 2 diabetes" OR "diabetes mellitus") AND ("mobile health technology" OR "mobile health app" OR "telemedicine" OR "mhealth") AND ("in-person care" OR "in-person visits" OR "in-person encounters" OR "in-person visits")