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This paper was prepared while the author was a staff member in the Methodology in Sociology training program at the University of Wisconsin, a project funded by the Institute of General Medical Sciences, N.I.H. The author is grateful to Hubert M. Blalock, Jr., and Otis Dudley Duncan for comments and criticisms of an earlier draft. They are in no way responsible for remaining errors, and the author is solely responsible for the general orientation and tone of the exposition.
David R. Heise (Wed,) studied this question.