Questions on migration starting from question 22 are household members related questions. Among theses members are individuals who were interviewed and others who were not interviews. Both populations are identified by their household identification and their number in the household schedule. The kids' file contains only kids whose mothers were interviewed. These kids are all identified by their sequential number in their mother's birth history and some of them by their number in the households schedule. Depending on what population target you are addressing, you will be using either their identification in the household and the file you will be using is the persons file (id is HHID and HVIDX). Kids are identified by their mothers identification CASEID which is HHID and HVIDX, and their ID which is BIDX.