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MIO: Reference Librarian
OR combines your synonyms and related terms to find documents that contain any or all of the words. It broadens your search and produces more results.
Your results would include all of the above documents.
AND combines your different concepts to find only the documents that contain all of the keywords. It narrows your search and produces fewer results.
Your results would include only the documents outlined in green.
NOT followed by a keyword will exclude all the documents that contain that particular word. It is a way of avoiding unrelated articles. Use it with caution as you may miss out on potentially useful material.
Your results would include only the documents outlined in green. Note that you may miss out on some relevant information in the document that talks about both heroin and drugs.