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Health Sciences Research Basics

Useful tips for research

Search operators and parentheses

Boolean operators


  • retrieves records that contain at least one of your search terms
  • e.g., (dog OR canine)


  • retrieves records that include all of your search terms
  • e.g., ultrasound AND tuberculosis


  • retrieves records that contain your first term but exclude your second term
  • e.g., dementia NOT alzheimer's
  • we do not usually recommend that you use NOT in your searches, as you may exclude relevant results

Note: Capitalize your operators as a matter of practice. In some platforms or search systems, it does not matter whether you enter them in uppercase or lowercase, but others (like PubMed or Google Scholar) require them to be in uppercase to work properly.

Proximity operators

  • Many bibliographic databases also allow use of proximity operators
  • The operator, if available, is dependent on the database and platform being searched
  • e.g., on the Ovid platform, adjn is used: primary adj3 care
  • Please see the database-specific operators and search fields


Use parentheses to set the order of execution of the Boolean logic. Parentheses work in most but not all systems (e.g., they work on the Ovid, PubMed, EBSCOhost, ProQuest, Scopus, and Web of Science platforms, but they are ignored in Google and Google Scholar)

e.g., (chest OR thorax OR thoracic) AND (imaging OR radiographs OR radiography)

Boolean operators - General tutorial

Boolean operators can be placed between your search terms to narrow or broaden a search, or to exclude search terms.

You will find below a brief video on how to use the Boolean operators:

Examples of search structure

Here's an example of four (incomplete but simple) searches that should end up with the same number of records but in which the Boolean logic and the search structure are either organized less or more efficiently or concisely.

For example, I am looking for literature on the use of ultrasound for diagnosing pulmonary tuberculosis.

(Depending on the databases, you may be using a different line syntax to combine line numbers, e.g., #1 OR #2 in PubMed, S1 OR S2 in CINAHL. This is a simplified illustration.)


Not recommended:

1. Ultrasound AND tuberculosis

2. Ultrasound AND tb

3. Ultrasound AND ptb

4. Ultrasonography AND tb

5. Ultrasonography AND tuberculosis

6. Ultrasonography AND ptb

1 or 2 or 3 or 4 or 5 or 6

Notice that it is hard to make sure I have combined all the terms comprehensively.


Recommended versions:

"Line by line" -- Easier to see if you have made spelling mistakes when you look at the number of records generated by each line; not all databases allow you to enter searches this way as it requires access to a search history:

1. ultrasound

2. ultrasonography

3. 1 OR 2

4. tuberculosis

5. tb

6. ptb

7. 4 OR 5 OR 6

8. 3 AND 7


"Block by block" -- One key concept or set of terms per line: More succinct but you must check even more carefully for spelling mistakes; once again, not possible in all databases (requires access to a search history):

1. (ultrasound OR ultrasonography)

2. (tuberculosis OR tb OR ptb)

3. 1 and 2


Complex searches will often mix these approaches, and that's okay.


"Single line" -- Many databases will allow for this type of entry, but you must check that parentheses (and intended application of Boolean operators, in some cases) are supported:

(ultrasound OR ultrasonography) AND (tuberculosis OR tb OR ptb)


It is important to note that this is an oversimplified example to illustrate the basic principles of structuring a search. Normally, a fully developed search strategy should include subject headings and textwords (with proximity operators, truncation, field codes, etc. as appropriate), e.g.,

(exp tuberculosis/ or mycobacterium tuberculosis/ or tuberculo*.mp. or (tb or ptb).ti,kf.) and (exp ultrasonography/ or (echogra* or echoscop* or echoso* or echotomogra* or POCUS or sonogra* or sonolog* or ultraso*).mp.)

Notice that it becomes more difficult to follow the logic in such a search, and care must be taken to ensure parentheses are used appropriately. A search can look fine on the surface but on closer examination may include parentheses in the wrong places, thus altering the results in critical ways.


For more advanced information on structuring searches, see:

Shokraneh, Farhad. Structure of search strategies for systematic reviews: Line by line versus block by block versus single-line. [Medium]. 2021 Jun 6 [Revised 2021 Jun 9].

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