Bangladesh 1993-1994 Demographic and Health ... - Measure DHS

C.2 FertilityThe low level of fertility reported in the BDHS and the extremely steep decline that it implies invitesa review of the quality of the data. There are a number of possible reasons why reported fertility rates couldbe erroneously low. The most likely explanation for a downward bias in fertility rates is that the dates of birthof some children were misreported so as to place them further back in time, either due to mothers' bias tomake the child older or to deliberate overstatement by the interviewer. Alternatively, some births could havebeen omitted from the birth histories of women either due to misreporting by the mother or to deliberateomission by the interviewer. Finally, the sample of women interviewed could be biased for some reasontowards women with low fertility, perhaps by over-representing urban women or those with more education.Pattems of overstatement of children's ages and/or omission of young children have been reportedin other DHS surveys (Amold, I990) and are generally believed to be due to a tendency of interviewers toreduce their workloads by reducing the number of children for whom the health questions must be asked.Overstatement of children's ages ("displacement") usually appears in the form of a sharp spike in the numberof births immediately prior to the cutoff year for eligibility of children for the health questions. Outrightomission of children is much more difficult to detect, as it is difficult to separate omission from a genuinedecline in fertility. However, a relatively high but steady number of births in the years prior to the cutoff datefor eligibility for the health questions, followed by an abrupt decline and a steady smaller number after thecutoff year implies that omission might be the cause. In the BDHS, the cutoff date for the health questionswas based on the Bangla, rather than the Western calendar; consequently, all children bom after April 1990were considered eligible for the questions in the health section.Table C.4 shows the number of births by Western calendar year and the percentage with completedates, the sex ratio at birth, the calendar ratio, and sex, according to survival status, while Table C.5 presentsthe same data using the Bangla year (April through March). The data do not show any indication ofdisplacement or transference of births across the boundary of the cutoff date. For example, the calendar ratiosin Table C.5 are very close to 100. If displacement had occurred, one would expect the ratio for April 1990-March 1991 to be low and that for 1989 to be high. The same data are shown graphically in Figure C. 1. Theyshow no evidence of displacement or of omission.Other evidence that might be expected to support a claim of displacement oromission of births is alsolacking. It is generally accepted that omission of births is not usually uniform but rather is selective of certainevents. Children who have died-----especially those who died when very young--and female children are oftensubject to greater omission than surviving children and males. However, infant mortality rates calculatedfrom BDHS data agree well with those from other sources and are not particularly low (see Figure 7.2).Furthermore, ratios of early deaths to those at older ages appear plausible (see Section C.3). Finally, sexratios are generally close to the expected value of 105 males per 100 females (Tables C.4 and C.5, column10), which implies that underreporting of births by gender was not a problem in the B DHS.The fact that there is no apparent evidence of omission or displacement of births in the BDHS datadoes not mean that the problem does not exist. Several researchers believe that backwards displacement ofbirth dates is a common source of error in Bangladeshi surveys (Cleland, 1993; Baimgi et al., 1991). Ifdisplacement of events (overstatement of children's ages) is not selective for particular children and if thereis no independent measure of children's ages, it is impossible to differentiate between displacement and truedeclines in fertility.168