Sources of error and bias in methods of fertility estimation contingent on the P/F ratio in a time of declining fertility and rising mortality

The most commonly used indirect fertility estimation methods rely on the use of the P/F ratio, first proposed by Brass. In essence, the ratio compares cumulated cohort fertility with cumulated period fertility on the basis of three, fairly strong, assumptions. First, that the level of fertility has remained constant over time. Second, that the age distribution of fertility has been constant; and third, that the fertility of women who do not survive to report their numbers of children borne does not differ from those who do survive. This paper interrogates what happens to the results produced by the P/F ratio method as each of these three assumptions is violated, first independently, and then concurrently. These investigations are important given the generally poor quality of census data collected in many developing countries, particularly those in sub-Saharan Africa, and the particular demographic dynamics resulting from the generalised HIV/AIDS epidemic in the region. The investigations suggest that using the P/F ratio for the age group 20-24 to scale the reported fertility schedule is more accurate than the Feeney method and marginally preferable to scaling using the average ratio for the age groups between 20 and 29, although it would overstate fertility while fertility is rising and for some time after period fertility peaks, reaching a maximum of around 10 per cent at the peak of period fertility. In addition differential fertility between HIV-infected and HIV-uninfected women has a trivial impact on the methods, even in an environment with a simulated highly generalised epidemic. 1Centre for Actuarial Research, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa. Tel.: + 27 21 650 5479. Fax: +27 21 689 7580. E-mail: [email protected] 2Centre for Actuarial Research, University of Cape Town, South Africa. E-mail: [email protected] http://www.demographic-research.org 1635 Moultrie & Dorrington : Error and bias in the P/F ratio method