Key points are not available for this paper at this time.
The author is grateful to Arthur S. Goldberger, Otis Dudley Duncan, and George Bohrnstedt for helpful comments concerning earlier papers on this topic. They share no responsibility for inadequacies of this presentation. Work on this project was partially supported by grant OE-5-10-292 from the Office of Education. Computer analyses were supported by funds from the Wisconsin Alumni Research Foundation and the National Science Foundation.
David R. Heise (Thu,) studied this question.