Variation is an instance of change or deviation. There is unwarranted variation in the practice of medicine and the use of medical resources in the United States. There is underuse of effective care, such as the use of beta-blockers for people who have heart attacks and screening of diabetics for early signs of retinal disease. There is misuse of preference-sensitive care, such as the choice between mastectomy and lumpectomy for early-stage breast cancer. And there is overuse of supply-sensitive care, such as admitting patients with chronic conditions like diabetes to the hospital, rather than treating them as outpatients.