Demographic and Health Surveys Methodology - Measure DHS

Notes and ConsiderationsAssumptionsMedians and means are based on current status of mothers of the births. The distributions of theproportions of births by month of birth of the child are analogous to the l x column of the synthetic lifetable. The basic assumption is that there has been no change in the proportions amenorrheic, abstaining,and insusceptible over time so that proportions that are calculated from births with small times since birthare the same as those of births that occurred earlier when those earlier births had those durations. For theshort period of 36 months, this assumption is very likely to be approximately correct. The proportions areassumed to be 1 at the time of birth and to decrease monotonically with time since birth. The time atwhich the proportions decline to 0.5 is taken as the median. The mean is taken similarly from the l xcolumn of the life table, which in this case is the series of proportions by time since birth. Because thesum of proportions equals 1, there is no need to divide the sum of the proportions times the width of theinterval.Because of the relatively small number of births in each month before the interview, the months are firstgrouped and then smoothed to reduce random fluctuations due to sampling variance.MeanTruncated Mean: Because of the limitation to births that occurred within the three years preceding thesurvey, the mean is truncated if there are mothers who are amenorrheic or abstaining longer than threeyears after their last birth. It is very unlikely that the proportions are more than negligible after 35 monthssince birth, except for those women who are not really postpartum amenorrheic or abstaining (more likelyfor older women whose postpartum amenorrhea blended into menopause or whose postpartum sexualabstinence blended into terminal abstinence). Therefore, it is felt that the truncated mean is very close tothe full mean.The measures are based on all births that occurred within the three years preceding the interview,including last and other births, surviving or not.Decisions on AlternativesCurrent status medians and means are used instead of measures based on the recall of durations becauseof the severe heaping (digit preference) on multiples of 3 and 6 months in the recall data.In the current calculation of median and mean durations, a woman can contribute more than once if shehad more than one birth in the three years preceding the survey. An alternative calculation would be tobase the medians and means on women rather than births. In this alternative, each woman is representedonly once, which is equivalent to durations based on only the last birth. Estimates of durations based onlast births (open interval durations) are thought to overestimate average durations of amenorrhea andabstinence since they almost always exceed those based on recall after other births (closed intervaldurations).Guide to DHS Statistics 76 Updated September 2006