official population data for large cities are unreliable and underestimate the real number of the urban population in large cities because many internal and external migrants live in big cities without being officially registered box this discrepancy between official demographic statistics on the one hand and social and demographic trends on the other could present issues for the effective provision of public services since budget transfers are based on official population numbers some large cities whose population has been underestimated may not receive enough funding for public services and to cope with the growing burden on urban infrastructure according to the law on migration internal migrants must register al their new place of residence