Comparisons of the quality of care that different types of doctors (for example, primary care doctors and specialists) provide for chronic illnesses, such as diabetes, must be done carefully. First, different types of patients may tend to see different types of doctors. For example, older patients or patients with multiple health problems may seek care from primary care doctors, who will care for all of their problems, instead of from endocrinologists, who would care only for the diabetes. In addition, not all doctors in a specialty practice alike. For this reason, comparisons of quality of care need to consider the fact that patients who see the same doctor are likely to get similar care, a concept known as “physician-level clustering.“