A clinical trial is one type of clinical research where a procedure or drug is intentionally administered outside the realm of standard medical practice to human subjects with the aim of studying its effect on the human body. This includes medications, operations, psychotherapy, physiotherapy, rehabilitation, nursing, restricted diets, and the use of medical devices. The comparative study of two or more treatments, involving the random assignment of treatments to patients, is considered a clinical trial even if the study includes approved drugs or medical devices. As the clinical trial is an experiment which involves human subjects, it is necessary to distinguish clinical trials from observational studies which collect outcomes when executing a study treatment as an ordinary treatment.