Find the Clinical Queries link in middle column under PubMed Tools.
Five study categories or filters are provided:
Etiology, Diagnosis, Therapy (default), Prognosis, Clinical prediction guides.
Two scope filters are provided:
Broad: Sensitive search – includes relevant citations but probably less relevant; will retrieve more (default)
Narrow: Specific search – will get more precise, relevant citations but less retrieval
See Search by Clinical Study Category and Find Systematic Reviews sections.
If doing a normal PubMed search, you can also use Limits to limit to certain Types of Articles, such as Clinical Trial, Meta-Analysis, Practice Guidelines, Randomized Controlled Trial, Review, Guideline, Systematic Review etc.