Once patients know they meet general criteria for a study, they go to the clinic site for a screening visit to determine whether they qualify officially. Typically at that visit, which can last one to several hours, they meet the trial coordinator and the principal investigator (usually a doctor) and are given a copy of the informed consent form to review. During this visit patients undergo a physical examination and a medical history review. Patients can also ask any questions they may have. Part of the screening may involve laboratory or diagnostic testing, but this is usually after the consent form is signed.