AND tells the database that all terms must be present in every result the database brings back. AND narrows your search; the more terms you combine using AND, the fewer results your search will retrieve. For instance: Neonatal Intensive Care Unit AND Noise AND Brain Development will only bring back articles that include all of those terms.
OR broadens your search. It tells the database to bring back results that have any of your search terms. The search brain development OR noise would bring back articles that discuss brain development, but not noise, and noise, but not brain development. OR is best used for synonyms or related terms. For instance: brain development AND (noise OR sound). Every article returned would discuss brain development, and either noise or the related term sound.
NOT excludes results that contain a specific term. It's good practice to use NOT sparingly, and generally in response to a problem in your search.
For instance, if you're looking for research on brain development and noise, and keep finding animal studies done on lab rats instead of humans, you could search for (brain development AND Noise) NOT rats
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