1. Hatcher RA, Trussell J, Stewart F, et al Contraceptive technology, 16th revised edn. New York: Irvington Publishers Inc. 1994: 109, 179–190.
2. Mosher WD, Martinez GM, Chandra A, et al Use of contraception and use of family planning services in the United States: 1982–2002. Advance data from vital and health statistics, no 350. Hyattsville, MD: National Center for Health Statistics. 2004.