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Multilingual Demographic Dictionary, second unified edition, English volume
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812
Migration statistics ^{1} are compiled to reveal the volume of migration, the direction of migratory movement and the characteristics of migrants. The accuracy with which each of these kinds of facts is ascertained depends upon the method of compilation, as most migration statistics consists of approximations and estimates rather than precise measurements. Direct measurement of migration ^{2} requires a system of recording movements as they occur. The most complete migration statistics are developed from population registers in which all changes of residence are recorded. They allow measurement of internal and of international migration, but are more satisfactory for the former than for the latter. In countries where these population registers do not exist, a certain number of administrative record systems which do not cover the entire population can be used for particular purposes. Thus, voter registration records ^{3}, social security records ^{4} tax-payers records ^{5} or dwelling records ^{6} may yield information on internal migration. In the case of overseas migration, statistics may be based on passenger lists ^{7} or manifests ^{7} of ships and aircraft. Counts of persons crossing a political frontier yield only very crude data; most of all in areas with much frontier traffic (803-2*) special steps must be taken to distinguish migrants from travellers ^{8}, who do not change their place of residence, and persons in transit 801-11). The number of visas ^{9} or entry permits ^{9} granted and the number of residence permits ^{10} or labor permits ^{11} issued may also be used as an indication of the migration of foreign nationals.
- 8. Traveller, n. - travel, v. - travel, n.: the process of travelling.
- 9. In certain countries residents who wish to travel abroad are required to obtain exit permits or exit visas, records of which may serve as a source of information on migratory movements.
813
Information collected in censuses and surveys allows the development of statistics on migrants ^{1}. Depending on the questions asked, these usually include statistics on in-migrants ^{2}, statistics on out-migrants ^{2} and place-of-birth statistics ^{3}. This approach has limits for the study of international migration; emigrants cannot be studied, whereas immigrants are known, whatever their country of origin.
814
Where it is not possible to determine migration directly, indirect estimates of net migration may be obtained by the residual method ^{1} or method of residues ^{1} in which the change in population between two dates is compared with the change due to natural growth; the difference between the two figures is attributed to migration. The vital statistics method ^{2} consists of computing the difference between total population change, as assessed from two censuses, and natural increase (701-7) during the intercensal period. The survival ratio method ^{3} is commonly used to estimate net migration by age; it does not require actual death statistics. Survival ratios may be derived either from life tables or from the comparison of successive censuses, and they are applied to a sub-population in one census to give expected numbers by age at the time of the other census. A comparison between the observed and the expected population may be used to estimate the balance of migration by age for the subpopulation. When place-of-birth statistics ^{4★} (813-3) by age and current residence are available in two consecutive censuses, it is possible to make indirect estimates of migration streams.
- 2. The equation showing that the difference between total population change and natural increase is equal to migration has sometimes received the name of balancing equation. In order to use it for the estimation of net migration, one must assume that omissions (230-3) and multiple countings (230-5) are equal for both censuses.
- 3. The major variants of this procedure are called the life table survival ratio method and the national census survival ratio method. In the forward survival ratio method, the population at the beginning of an intercensal period serves to estimate the expected population at the end of the period, and the procedure is reversed in the reverse survival ratio method; the average survival ratio method combines these two approaches.
815
The generic term migration rate ^{1} refers to any rate which measures the relative frequency of migration within a population. Unless indicated otherwise these rates should be taken as annual migration rates ^{2}. They may be obtained as the ratio of the average annual number of movements during a certain period, to the average population of the period. An annual rate of net migration ^{3} and an annual rate of total migration ^{4} are calculated in a similar fashion by using the appropriate information on net and total migration. An index of migration effectiveness ^{5} or effectiveness index ^{5} is calculated as the ratio of net migration to total in- and out-migration. The range of the index is from zero, when arrivals and departures are equal in number, to one, when migration is entirely one way.
- 2. Other denominators may serve to compute the rate, such as the population at the beginning or the end of the period, or the number of person-years lived by the population of the area.
- 5. Also: Index of migration efficiency or efficiency index.
816
Proportions of migrants ^{1} can be obtained by relating the number of migrants during a period to the population to which or from which they are migrating. When the proportion of out-migrants ^{2} is obtained by dividing the number who reported moving out of the area by the population residing in the area at the beginning of the period and alive at the end, this index measures the probability of moving for the population at risk, and among other uses, it can be used in the preparation of population projections where migration is accounted for separately. But other populations are often used in practice as denominators to compute proportions of migrants. Similarly, the proportion of in-migrants ^{3} is sometimes obtained by dividing the number of in-migrants in an area during a period, by the population of the area at the end of the period; but the denominator could also be the population at the beginning of the period, or the average of the beginning and end populations. The proportion of lifetime in-migrants ^{4} can be derived from information on the place of birth, dividing the number of persons born out of the area by the enumerated population of the area. The proportion of lifetime out-migrants ^{5} can be obtained by dividing the number of persons in a country living outside of their area of origin, either by the total number of persons born in that area, or by those among them who still live there. When such characteristics of the migrants as age (322-1), occupation (352-2) or level of education (342-1) are known, indices of migration differentials ^{6} are used to contrast the migrants and the rest of the population of destination. The index is equal to the quantity 1 minus the ratio of the proportion of migrants in the population having the characteristic studied to the proportion of migrants in the whole population. The index of migration differentials is equal to zero when the population with the given characteristic has the same migration behavior as the rest of the population. The term selectivity of migration ^{7} indicates that the comparison is between the in-migrants and the population from which they were drawn, at the area of origin (801-4). When comparing the characteristics of the in-migrants to those of the population at the place of arrival (801-5) the term differential migration ^{8★} or migration difference ^{8★} is sometimes used.
- 7. For example, the selectivity of migration from Mexico was decreasing because differences in characteristics between migrants and nonmigrants fade over time; also the origins of Mexican immigrants was increasingly diverse because of the spread of Mexican migration networks in the USA.
817
Longitudinal migration analysis ^{1} requires information on the successive moves of an individual over time, information which is normally available only from population registers (213-1) or retrospective surveys (203-8). Several refined measures of migration are available from this type of data, such as a first migration probability ^{2}, defined as the probability that a group of non-migrants ^{3} aged x will be involved in migration for the first time before reaching age x + n . These probabilities can be used to calculate a non-migrant table ^{4}. The latter, when combined with a life table (432-3) will lead to a double decrement survivorship schedule of non-migrants ^{5}. Similarly, migration probabilities by order of move ^{6} can be computed, as well as the proportion of migrants of a given order who have not gone on to make a subsequent move within a certain migration defining interval. The all orders migration rate ^{7} is the ratio of moves of all orders in a year to the average population size of the cohort (117-2) over the year. The cumulation of these rates for a cohort up to a given date provides an estimate of the mean number of moves ^{8} in the absence of mortality. A survivorship schedule can be combined with an age-specific all orders migration table ^{9} to estimate the average number of remaining moves for an individual of a given age, given the prevailing mortality.
818
In studying migrants between two areas during a period, one commonly used measure is the index of migration intensity ^{1}, obtained by dividing the number of migrants from area A to area B by the product of the number of inhabitants in B at the end of the period, and the number of inhabitants of A at the beginning of the period who are still alive at the end. This index, divided by the ratio of the total number of migrants to the square of the population of the country, yields a migration preference index ^{2}. When the numerator is restricted to the net stream of migration, the resulting measure is called an index of net velocity ^{3}. The effectiveness of migration streams ^{4} is measured by relating the absolute value of the net migration stream to the gross interchange (805-10).
- 1. This index can be interpreted as the probability that two individuals alive at the end of the period selected randomly, one among those residing in area A at the beginning of the period, and the other among those residing in B at the end of the period, will be identical. The availability of data may impose various other denominators.
819
Migration models ^{1} fall in two broad categories. The first relates migration streams (803-9) between two areas to social, economic or demographic variables. These variables are often classified as push factors ^{2} when they characterize repulsion ^{2} from the area of origin, as pull factors ^{3} resulting in attraction ^{3} to the area of destination, and as intervening obstacles ^{4} between the two areas. The simplest of these models are gravity models ^{5}: the streams between the two areas are directly proportional to the size of their population, and inversely proportional to the distance ^{6} between them, raised to a certain power. Other models consider that the streams are proportional to the opportunities in the area of destination, and inversely proportional to intervening opportunities ^{7} between origin and destination. Models in the second broad category are stochastic models (730-5) and refer to individuals rather than to populations; they link the probability of migrating to a certain number of personal characteristics such as age or the previous history of migration.
- 5. Or Pareto-type models.
- 6. Distance can be measured in diverse ways: a straight line, the route, the number of intervening areas, etc.
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