23 Spector WD, Katz S, Murphy JB, Fulton JP: The hierarchical relationship between activities of daily living and instrumental activities of daily living. J Chronic Dis 1987; 40: 481-489. 24 Kemper P: The use of formal and informal home care by the disabled elderly. Health Serv Res 1992; 27: 421-451. 25 Hellstrom Y, Hallberg IR: Perspectives of elderly people receiving home help on health, care and quality of life. Health Soc Care Community 2001; 9: 61-71. 26 Melzer D, Lan T-Y, Tom BDM, Deeg DJH, Guralnik JM: Variation in Thresholds for Reporting Mobility Disability Between National Population Subgroups and Studies. J Gerontol A Biol Sci Med Sci 2004; 59: 1295-1303. 27 Guralnik JM, Simonsick EM, Ferrucci L, Glynn RJ, Berkman LF, Blazer DG, Scherr PA, Wallace RB: A short physical performance battery assessing lower extremity function: association with self-reported disability and prediction of mortality and nursing home admission. J Gerontol 1994; 49: M85-94. 28 Ensrud KE, Nevitt MC, Yunis C, Cauley JA, Seeley DG, Fox KM, Cummings SR: Correlates of impaired function in older women. J Am Geriatr Soc 1994; 42: 481-489. 29 Gill TM, Williams CS, Tinetti ME: Assessing risk for the onset of functional dependence among older adults: the role of physical performance. J Am Geriatr Soc 1995; 43: 603-609. 30 Jette AM, Branch LG: The Framingham Disability Study: II. Physical disability among the aging. Am J Public Health 1981; 71: 1211-1216. 31 Guralnik JM, Ferrucci L, Pieper CF, Leveille SG, Markides KS, Ostir GV, Studenski S, Berkman LF, Wallace RB: Lower extremity function and subsequent disability: consistency across studies, predictive models, and value of gait speed alone compared with the short physical performance battery. J Gerontol A Biol Sci Med Sci 2000; 55. M221-231. 32 Manton KG: A longitudinal study of functional change and mortality in the United States. J Gerontol 1988; 43: S153-161. 33 Jylha M, Guralnik JM, Balfour J, Fried LP: Walking difficulty, walking speed, and age as predictors of self-rated health: The women's health and aging study. J Gerontol A Biol Sci Med Sci 2001; 56: M609-617. 34 Cress ME, Schechtman KB, Mulrow CD, Fiatarone MA, Gerety MB, Buchner DM: Relationship between physical performance and self-perceived physical function. J Am Geriatr Soc 1995; 43: 93-101.