Measuring potential fertility through null segments: An exploratory analysis

Abstract

The demographic effectiveness of a family planning program must measure the difference between actual fertility and the "gross" potential fertility, i.e., the natural fertility which would have been achieved without use of contraception. The various methods of measuring "gross" fertility which have been used are described. It is understood that users of contraception are never a random sampling of the general population; they tend to be higher in fecundability and lower in proportion of sterile. For this reason, the best strategm of measurement involves utilization of preacceptance fertility rates over some given period of time among those reporting no contraceptive usage during that period. A procedure of measurement is proposed whereby natural fecundability is estimated from null segments. Possible biases are taken into account. Application of the method is made to 2 sets of data collected in Taiwan. Results of the application of the method to the Taiwanese data are tabulated. Fecundability functions derived from unrestricted null segments are different from those constructed from restricted subsets of "closed" null segments. Choice of whether to use restricted or unrestricted null segments will depend on the nature of the data available.