Think of a principal investigator (PI) of a clinical trial as similar to the producer of a film. The PI, usually the lead writer on the research grant, is in charge of the whole thing, including designing and carrying out the protocol and collecting, analyzing and reporting the data. Most universities have published guidelines describing the roles and responsibilities of the PI and co-investigators. Examples include this one from the University of Massachusetts Amherst and this one from Washington University in St. Louis, Missouri.
