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Just like any automated process, automatic term mapping is not perfect. Here are a couple examples and how to fix these errors:
Nursing
A search for "nursing" in PubMed yields results for nursing as a profession and breastfeeding.
To fix the problem, highlight the part of the search that you want to conduct and paste it into the query box, then re-run it.
Patient Outcomes
A search for patient outcomes:
PubMed broke apart the terms. To fix the problem, put the search terms in quotations. This will search for the terms together, but will also turn off PubMed's ability to do automatic term mapping.
For even better results, add alternative terms and re-run the search.