To specifically focus on finding research articles, remember these 2 search strategies:
1) Advanced Search - Research Article Check Box: In the Advanced Search, checking the box next to Research article will help with retrieving *mostly original or primary resarch. You may also find other research that is secondary or an occasional article that is incorrectly categorized.
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2) Advanced Search: Clinical Queries: Be sure to select the query type that is most appropriate - Therapy (treatment), Prognosis (likely outcome), Causation (etiology), etc. Best Balance is usually a good default choice.
PubMed's Clinical Queries can be found under Tools from PubMed's Homepage. Be sure to select the Query type most appropriate for the information that you're seeking. *You may pull up some articles that are not original or primary research, so please double check that the article is truly a research article.