In many countries, the term principal investigator (PI) refers to the holder of an independent grant and the lead researcher for the grant project, usually in the sciences, such as a laboratory study or a clinical trial.
The phrase is also often used as a synonym for "head of the laboratory" or "research group leader".
[1] A co-investigator (Co-I) assists the principal investigator in the management and leadership of the research project.
[2] In the context of United States federal funding from agencies such as the National Institutes of Health (NIH) or the National Science Foundation (NSF), the PI is the person who takes direct responsibility for completion of a funded project, directing the research and reporting directly to the funding agency.
[citation needed] In the context of a clinical trial, a PI may be an academic working with grants from NIH or other funding agencies, or may be effectively a contractor for a pharmaceutical company working on testing the safety and efficacy of new medicines.