Clinical investigation is synonymous with clinical study and refers to any study involving human volunteers (also called participants) that is intended to add to medical knowledge. It is conducted under established guidelines by trained medical professionals. There are two main types of clinical studies, clinical trials and observational studies. A clinical trial tests or evaluates an intervention — medicine, medical device, activity, or procedure — in people. In an observational study, researchers observe participants on their current treatment plan and track health outcomes.