The Nurses' Health Study II was established in 1989 by Dr. Walter Willett, who has been its principal investigator since inception. The focus of the study was women's health, especially the long term adverse effects of oral contraceptives. The sample population contained females within the age range of 25-42, employed as nurses, from 14 U.S. States. Data collected included the brand of pill and length of use. Over time, the study expanded to include information on basic practices and measurements of health, such as exercise practices and food intake.