M.D.s are sometimes referred to as allopathic physicians. As needed, medical doctors might refer patients to specialists who focus on specific medical areas, such as anesthesiology, cardiology, psychology and internal medicine. Specialists are experts in their field and complete additional residency training, and become board certified in their specialty. Doctors may work long and unpredictable hours dictated by the needs of their patients. Additionally, doctors may need to travel amongst various locations, such as offices, hospitals and clinics, in order to provide patient care. Doctors who practice in healthcare organizations or groups have less work independence but may obtain more time off as a result of patient coverage.