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2. 2. U.S. Department of Commerce, Bureau of the Census (1982 ) Preliminary Estimate of the Population of the United States, by Age, Race, and Sex: 1970-1981 . U.S. Government Printing Office.
3. 4. E.W. Busse, E. Pfeiffer, E. W. Busse, and E. Pfeiffer (1977 ) Behavior and Adaption in Late Life () 2nd edition , p.102 . Little, Brown and Company, Boston.
4. 5. D.S. Woodruff ,op. cit., p.51 .