Demography, Vol. 28, No.2, May 1991 Individual-Level Sterility: A New Method of Estimation with Application to Sub-Saharan Africa VIla Larsen Department of Sociology SUNY Stony Brook Stony Brook, NY 11794-4356 Jane Menken Department of Sociology and Population Studies Center University of Pennsylvania 3718 Locust Walk Philadelphia, PA 19104-6298 This paper extends work on measures of population proportions sterile to propose a new estimator of an individual woman's age at sterility and consequently her sterility status at given ages. Accuracy and reliability, examined in a simulation study, appear satisfactory. From World Fertility Survey data for five African countries, the proportions sterile by age estimated by the individual measure and by the population estimator are almost identical. Cameroon and Kenya show substantial variation in prevalence and incidence of sterility across ethnic groups and by number of marriages. Unexpectedly, the evidence suggests that sterility increased from 1960 on in Kenya and remained unchanged in Cameroon. In earlier work (Larsen and Menken 1989) we developed a method for estimating sterility in a population from a sample of women, not all of whom had reached the end of their reproductive span. Neither that procedure nor those from which it was derived (Henry 1965; Leridon 1977; Trussell and Wilson 1985; Vincent 1950) measure the sterility status of a particular woman; rather, they estimate the age-specific proportions sterile. Currently available individual-level methods are unsatisfactory for a variety of reasons described in our earlier paper. Yet if a woman's sterility status at any given age and her age at the onset of sterility could be determined with reasonable accuracy, the study of the determinants and the differentials in sterility both within and between populations would be feasible, and sterility could be treated as a predictor that is related to, or affects, dependent variables of interest. Therefore we attempt the extension of our earlier work to the individual level when information is based either on complete reproductive histories (when all women are followed until age 50) or on incomplete histories (such as those obtained from women of reproductive age on a particular date when a survey is taken). This effort is particularly timely; increasingly, data are becoming available from populations showing evidence of high levels of sterility, such as those of sub-Saharan Africa; there, for example, more than 20% of women aged 25 in Cameroon are sterile (Larsen and Menken 1989), and childlessness is common in various populations, such as those of Ethiopia (Mammo and Morgan 1986) and Zaire (Tabutin 1982). Surveys Copyright © 1991 Population Association of America 229