Key points are not available for this paper at this time.
Barbara Vaughan, James Trussell, Jane Menken, Elise F. Jones, Contraceptive Failure Among Married Women in the United States, 1970-1973, Family Planning Perspectives, Vol. 9, No. 6 (Nov. - Dec., 1977), pp. 251-258
Vaughan et al. (Tue,) studied this question.