Demographic and Health Surveys Methodology - Measure DHS

where p i is the proportion amenorrheic, abstaining, or insusceptible for the first group, where theproportion is below 0.5, p i-1 is the proportion amenorrheic, abstaining, or insusceptible for the precedinggroup, m i-1 is the midpoint value for the preceding group, and w i is the time width of the group taken asthe difference between the midpoint value of the current group and the preceding group.For all groups except the first, the midpoint values are ½ month above the lower limit. This midpointvalue comes about because the time since birth is calculated as the difference between birth dates and dateof interview that are accurate for month only (day of month is not ascertained).For example, if the date of interview were April 2001, the interview could have occurred at any timeduring the month, from the 1 st to the 30 th . The same holds true for a birth that occurred in January 2001, atany time between the 1 st and the 31 st of the month. Thus, the difference in time between the date of birthand the date of interview could be between 60 days and 120 days. Assuming a constant distribution byday of month for interviews and for births, the midpoint is 90 days or three months, which is the value ofthe difference in the century-month codes of the dates. The midpoint value for the group of the differenceof 2 months and 3 months together is therefore 2.5.The first group is special. The value of the previous group is assumed to be 100 percent since all womenare assumed to be amenorrheic and abstaining on the day of birth. Moreover, births that occur in themonth of interview can only come before the date of interview, rather than on any day of the month ofinterview. Based on simulations, 0.75 is chosen as the midpoint value for the group < 2 months (i.e., 0–1).The midpoint of the previous group is 0 (day of interview).An example of the median calculation is given below:Example:MeanGroup P i Midpoint Width