How to Use Boolean Operators in Research

Boolean operators combine search terms logically: OR broadens a search by joining synonyms, AND narrows it by requiring that all concepts appear, and NOT excludes records containing a term. Using them with brackets makes your logic explicit and reproducible.

What each does

PubMed needs operators in capitals. PubMed processes operators from left to right unless brackets change the order, so use brackets.

Nesting with brackets

Compare stroke OR "cerebrovascular accident" AND rehabilitation (ambiguous) with (stroke OR "cerebrovascular accident") AND rehabilitation (clear). The brackets say: first combine the synonyms, then require the second concept.

Illustrative example

Invented for teaching. A search for children with asthma and vitamin D: (child* OR paediatric OR pediatric OR adolescent*) AND (asthma OR wheez*) AND ("vitamin D" OR cholecalciferol). Each bracket is one concept; OR sits within brackets and AND sits between them.

Why NOT is risky

NOT can remove relevant papers that mention the excluded term in passing. For example NOT animals may remove human studies that also report animal work. Prefer a filter or a manual screening step, and justify any use of NOT.

Common pitfalls

• Forgetting brackets, so that the search does not mean what you intended.
• Using AND between synonyms, which shrinks results to almost nothing.
• Using different logic across databases without checking each interface.

Key takeaways

• OR within a concept, AND between concepts.
• Use brackets to control logic and avoid NOT unless necessary.
• Check how each database interprets syntax.

This page is general educational information about research methods. It is not medical, legal or regulatory advice; follow your institution's ethics board and your target journal's instructions.