References
- Bureau of Labor Statistics 1996. Economic Report of the President. Table B-42, p. 329 and Table B-31, p. 316.
- Economic Outlook June 1993, Table 15, p. 215.
- Economic Outlook Dec. 1995, Table 13, p. 25.
- Economic Report of the President, 1997. Cf. Table B-l, p. 300, and Table B-81, p. 394.
- Government Printing Office, Bureau of Census 1975.
- Statistical Abstracts of the United States, various years.
- Historical Statistics of the United States, Colonial Times to 1970, Series H pp. 238–244, p.350 (1937-1970).