Principal investigator (PI)

The lead doctor or researcher in charge of running a trial and looking after the safety of everyone in it.

The principal investigator — often shortened to PI — is the person in charge of a clinical trial at a given site. Usually a doctor or senior researcher, the PI makes sure the study follows its protocol, that the team is qualified, and above all that the people taking part are kept safe and treated fairly.

If you join a trial, the PI is ultimately responsible for your care within the study, though you’ll often work day to day with study nurses and other staff on the team. The PI also answers to outside watchdogs like the Institutional Review Board. When you have a hard question about the study, the PI is the person accountable for a clear answer.