1. National Center for Health Statistics. http://www.cdc.gov/nchs/data/hus/tables/2003/03hus027.pdf
2. National Center for Chronic Disease Prevention and Health Promotion. http://www.cdc.gov/nccdphp/aag/aag_aging2004_access01.htm
3. Administration on Aging. http://www.aoa.gov/prof/Statistics/statistics.asp/
4. Recent trends in disability and functioning among older adults in the United States;Freedman;A systematic review,2002