Outside of longitudinal surveys, vital registration provides the main source of information on events such births, deaths, marriages and migrations. In the US, newborns are issued a birth certificate, death certificates are issued after one's death, visitors to the US cannot leave the country without returning their I-94 card on which their age and nationality is recorded, marriages require marriage licenses etc... Demographers compile these records to learn about flows and various types of entries and exits that may affect a population.
Only a few countries however have nearly complete vital registration systems. It usually takes a well organized administrative structure to capture events occurring at the local level, and lack of motivation, under-staffing or inadequate resources among registration agencies may lead to large numbers of omissions and errors. In developing countries, in particular, such systems are either non-existent or seriously incomplete.
Often, errors and omissions in vital registration data are concentrated on a few groups or areas: deaths among neonates occurring immediately after birth, for example, may frequently go unrecorded; registration tends to be better in urban than in rural areas, and -in some countries- can be gender-biased if the status of women in the community is low or if couples express a strong preference for male children. In the US, several studies have shown that the quality of vital registration varied greatly across racial groups, with the registration of vital events much less complete and accurate among blacks.