NUcat is the library's online catalog that can help you find items held by all the Northwestern libraries such as the Main Library in Evanston, Schaffner, and the law school library.
The most useful subject headings for your research may be:
Clinical Trials as Topic
Clinical Trials as Topic--Methods
Clinical Trials as Topic--Standards
Research Design (Medical subject heading)
Statistics as Topic (Medical subject heading)
Biostatistics (Medical subject heading)