The Normative Aging Study (NAS) is a longitudinal study which studies the effects of aging on various health issues. The ongoing study was established in 1963 by the United States Department of Veterans Affairs. The initial sample was 2,280 men now with an average age of 72 years (mean age at entry was 42 years). Most participants are veterans from WWII and the Korean War. Participants in the study have undergone medical examinations every three to five years, also answering questions about behaviors affecting health.