PRECALBIDX (V235) provides the birth index number (see BIDX for more information) of the last child born to the woman before the start of the survey's birth calendar. For the majority of samples, PRECALBIDX reports the birth index number of the last child born prior to 5 years before the survey. For a minority of samples, the reference period is 3 or 4 years before the survey. See Sample Description [URL omitted from DDI.] information on the universes for children included in the surveys to learn the specific reference period for each survey.