Los puntos clave no están disponibles para este artículo en este momento.
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.