As such it is regarded as a Driving Force indicator. The definitions of immigrant and emigrant used by different countries and even for different data sources within a single country vary considerably, thus compromising the comparability and interpretation of the indicator. The data are often poorly measured restricting the usefulness for modelling purposes. Illegal immigrants are not captured by the census or survey statistics. They vary, however, in the type of data they collect and the way in which the information obtained is coded and tabulated.
Some countries record the urban or rural nature of the place of residence involved.
However, net rural-urban migration is more likely to be derived from indirect estimation procedures than directly from census data. In general, data on internal migration gathered by censuses remain underexploited and there is no comprehensive source of information of net migration rates between different units within countries, except for countries with a population register. Direct and comprehensive data on international migration movements are not available for most countries or areas of the world. Population registers are the data sources most likely to yield the information needed to calculate the net migration rate, but only a few developed countries maintain such registers. Immigration statistics derived from the administrative procedures involved in admitting foreigners for residence do not provide good measures of flows nor are they sufficiently comprehensive to permit the estimation of migration rates as they do not reflect the migration of citizens.