How is migration derived? (cont.)
Place of birth
Questions which ask where each individual was born allow for measurement of the number of lifetime in-migrants or immigrants in an area. Those who were born in the area in which they are enumerated are considered non-migrants, while those who were born somewhere other than the area in which they are enumerated are considered migrants. This data can be group according to place of origin, giving an estimate of the major migration flows into an area.
One disadvantage of this method is that it will miss return migrants in its estimation of lifetime migrants. A person may be born in an area, move away for a period of time and then return (indeed this is common). However as they live in the place which they were born, this interim migration information is lost.
A second disadvantage is the lack of information on the timing of the migration. A person may have migrated into an area a few days ago or a few decades ago. This makes it difficult to estimate current migration flows. An area may have experienced a high volume of migration flows in the past, and thus accumulated a large number of lifetime migrants. However this does not tell us whether there are still large volumes of migration flows today.