Last Revised: January 30, 2007
are derived by the method of extinct generations for all cohorts that are extinct^{4 }(shown in green in Fig. 1) and by the survivor ratio method^{5 }for nonextinct cohorts who are older than age 90 at the end of the observation period (shown in blue in Fig. 1). For nonextinct cohorts aged 80 to 90 at the end of the observation period, population estimates are obtained either from another source or by applying the method of intercensal (or postcensal) survival.
Figure 1. Methods used for population estimates
Age Max
B
C
90
80
A
A
0
year t_{0 }
year t_{n }
Time
A  Official estimates / intercensal survival B  Extinct cohorts C  Survivor ratio, SR90+
4. Exposuretorisk. Estimates of the population exposed to the risk of death during some age time interval are based on annual (January 1st) population estimates, with a small correction that reflects the timing of deaths during the interval. 6
5. Death rates. For both periods and cohorts, death rates are simply the ratio of deaths to exposuretorisk in matched intervals of age and time. For broader intervals of age and/or time (whether time is defined by periods or cohorts), death rates are always found by pooling deaths and exposures first and then dividing the former by the latter. 7
6. Life tables. Period death rates are converted to probabilities of death by a standard method.^{8 }Cohort probabilities of death are computed directly from estimates of deaths and exposure,
4 5 6 For details regarding the extinct cohort method, see pp. 2729 of the Methods Protocol. For more information regarding the survivor ratio method, see pp. 2932 of the Methods Protocol. See pp. 3233 of the Methods Protocol for the formulas for calculating exposure estimates for periods (Equation 49) and cohorts (Equation 52). For details regarding the calculation of death rates, see pp. 3234 of the Methods Protocol. See pp. 3539 of the Methods Protocol for the formulas used to calculate period life tables. 7 8

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