The data are from the National Health and Nutrition Examination Survey (NHANES). NHANES is a national survey designed to collect information on the health and nutritional status of the U.S. civilian noninstitutionalized population through in-home interviews and physical examinations. The health examination is performed in a mobile exam center (MEC). Starting in 1999, the NHANES has been conducted continuously. More detailed information can be found at the NHANES homepage