Fertility Histories: With and Without Restrictions – An Analysis of PLM Data

During the last decade, a large number of countries participated in the World Fertility Survey but few of them collected fertility histories that were not partially restricted. In a majority of the cases information on the duration of breast-feeding and contraceptive use was restricted to the last closed and the open intervals only. These restrictions on the fertility histories have raised many questions about the possibility of sample selection bias in the results. A number of researchers in the developed countries have used these surveys for analyzing the effects of breastfeeding and contraception on the length of birth intervals. They have acknowledged the possibility of a bias in the results and have taken measures to minimize these potential biases. In this paper we will initially discuss the ways in which biased histories produce a biased sample of births. Later we wi11 evaluate the effects of the restrictions by using the fertility data from the Population Labour Force and Migration (PLM) Survey. This data contains detailed reproductive histories of 9416 currently married women having 38,746 children selected from 11 ,000 households sampled in the PLM survey.