Summary
- Migration is the movement of individuals or groups between locations when there is a change of usual residence to a different administrative or political unit.
- Gross migration is the total flow of migrants across a border, i.e. in-migrants + out-migrants, or in the case of international migration, immigrants + emigrants.
- Net migration is the difference between the inward and outward flows of migration, i.e. in-migrants – out-migrants or immigrants -- emigrants.
- Charting the distribution of the population across space is important as the migration of populations to urban areas is leading to growing urbanisation of the global. The meaning of urban and rural can vary between countries and there is no simple and unique definition for distinguishing urban and rural populations.
- Population density, usually calculated as the number of people per square kilometre, or per square mile is useful to get an idea about the spatial distribution across the territories of countries or regions.
- Censuses or surveys are the primary source of migration data – specifically questions about: place of birth, place of last residence, duration of residence in current location, place of residence at a specific date prior to the census (e.g. five years previous).
- It is possible to estimate the net migration between two consecutive censuses. One of the most common approaches to estimate net migration is the Survival Ratio Method to apply a set of survival ratios to the population living in an area at the date of the first census, deriving the number of persons expected to survive at the second census.
- The out-migration rate is the probability of moving from a given origin to a specific destination in the given time period. The population exposed to the risk of moving, for migration streams, will be the population residing in the origin area and who survived to the census or survey.
- The in-migration rate is more complicated and can be ambiguous as it refers to the area of destination.
- The difficulty of choosing a base population to calculate an appropriated net migration rate is problematic since the population at risk of in- and out-migration can be very different in size
- Migration can affect the age and sex structure of the populations of origin and destination both directly and indirectly.
- The age pattern of migration obeys certain regularities: high in infancy, low in early teens, rapid rise during teens and early twenties, peak between ages 20 and 25, gradually decline to early 50s and then old age variants (constant; gradually rising; retirement peak).
- Models of migration by age can be used for graduating and adjusting missing data, modelling non-existent data, and for projection and population accounting. Life tables can be constructed for mortality and for transition into and out of each region. From these, it is possible to calculate not only overall life expectancy, but also the expected number of years lived in each region, given the region of birth.