Estimation of Fertility by Reverse-survival Methods

1. Meaning ofreverse survival In a closed population, children currently aged x are just the survivors of the births that occurred x years ago. From this fact it is easily inferred that the number of births occumng x years ago can be estimated by using life-table survivorship probabilities to "resurrect*' numerically those no longer present among the population aged x . This method of estimation is known as ''reverse survival" or "reverse projection" because the population how aged x is "survived" or "reverse-projected" to age x t by moving it, with a suitable life table, t years into the past. It is immediately evident that if the single-year age distribution of a population enumerated at to is available, it is potentially possible to estimate the number of births occurring during each of the 15 or 20 years preceding 10. Methods that exploit this possibility are described in this chapter. However, before proceeding with their description, it is worth noting that they are all heavily dependent upon the accuracy of the reported age distribution of the population being studied. Errors in age-reporting or differential completeness of enumeration affecting certain age groups, especially the younger ones, are certain to bias the estimates obtained. Because these types of deficiencies are all too frequently characteristic of the data sets available, reverse-survival methods are often ineffective in producing reliable fertility estimates. Their usefulness depends mainly upon the fact that they often provide independent fertility estimates which can be used to assess the plausibility of those obtained by other means.