Decrement tables, also called life table methods, are used to calculate the probability of certain events.
Birth control 
Life table methods are often used to study birth control effectiveness. In this role, they are an alternative to the Pearl Index.
As used in birth control studies, a decrement table calculates a separate effectiveness rate for each month of the study, as well as for a standard period of time (usually 12 months). Use of life table methods eliminates time-related biases (i.e. the most fertile couples getting pregnant and dropping out of the study early, and couples becoming more skilled at using the method as time goes on), and in this way is superior to the Pearl Index.
Two kinds of decrement tables are used to evaluate birth control methods. Multiple-decrement (or competing) tables report net effectiveness rates. These are useful for comparing competing reasons for couples dropping out of a study. Single-decrement (or noncompeting) tables report gross effectiveness rates, which can be used to accurately compare one study to another.
See also 
- Kippley, John; Sheila Kippley (1996). The Art of Natural Family Planning (4th addition ed.). Cincinnati, OH: The Couple to Couple League. pp. 140–141. ISBN 0-926412-13-2. which cites:
- Trussell J, Hatcher RA, Cates W et al. (1990). "A guide to interpreting contraceptive efficacy studies". Obstetrics and Gynecology 76 (3 Pt 2): 558–567. PMID 2199875.
- Potter RG (1966). "Application of life table techniques to measurement of contraceptive effectiveness". Demography (Demography, Vol. 3, No. 2) 3 (2): 297–304. doi:10.2307/2060159. JSTOR 2060159. PMID 21318704.
- Trussell J (1991). "Methodological pitfalls in the analysis of contraceptive failure". Statistics in medicine 10 (2): 201–220. doi:10.1002/sim.4780100206. PMID 2052800.
- Trussell J, Grummer-Strawn L (1991). "Further analysis of contraceptive failure of the ovulation method". American Journal of Obstetrics and Gynecology 165 (6 Pt 2): 2054–2059. PMID 1755470.
- Sheps MC (1966). "Characteristics of a ratio used to estimate failure rates: occurrences per person year of exposure". Biometrics (Biometrics, Vol. 22, No. 2) 22 (2): 310–321. doi:10.2307/2528521. JSTOR 2528521. PMID 5961447.
- Trussell J, Kost K (1987). "Contraceptive failure in the United States: A critical review of the literature". Studies in family planning (Studies in Family Planning, Vol. 18, No. 5) 18 (5): 237–282. doi:10.2307/1966856. JSTOR 1966856. PMID 3318006.
- Trussell J, Grummer-Strawn L (1990). "Contraceptive failure of the ovulation method of periodic abstinence". Family Planning Perspectives (Family Planning Perspectives, Vol. 22, No. 2) 22 (2): 65–75. doi:10.2307/2135511. JSTOR 2135511. PMID 2189750.
- Trussell J, Strickler J, Vaughan B (1993). "Contraceptive efficacy of the diaphragm, the sponge and the cervical cap". Family Planning Perspectives (Family Planning Perspectives, Vol. 25, No. 3) 25 (3): 100–105, 135. doi:10.2307/2136156. JSTOR 2136156. PMID 8354373.