Overview
Migrationis the third process (with fertility and mortality) that governs populationchange. For most national populations, its contribution to population change issmall relative to those of births and deaths, but as the civil division ofinterest becomes smaller, the salience of migration typically becomes larger. Migrationdiffers from fertility and mortality not only in magnitude, but morefundamentally in the nature of the process. Migration involves moving acrosssome geographically-defined boundary, with the intent or result of changingplace of normal residence. Thus whereas a birth and a death are largelyunambiguous, a migration depends upon geographically-defined spatial units(civil divisions) and on intent or subsequent behaviour. A person can be amigrant to the analyst looking at change in provincial population but not amigrant to another analyst focusing on national population change. The firsttask, therefore, in any analysis of migration is to establish the geographicfocus of the study. A second task is to define what counts as a migration, asopposed to broader mobility. The issue is further confused by the existence ofseveral different types of migration. In addition to “ordinary” change of usualresidence, there are circular migration flows, daily or weekly commuter flows,seasonal flows and refugee flows, all with specific characteristics. Giventhese definitional issues, and the fact that migrations can effectively bereversed in terms of population stocks (unlike births and deaths), it is nosurprise that measurement is also complicated.
Apartfrom this, capturing data on migration is also more problematic. Althoughdeveloping countries often lack complete systems of birth and deathregistration, completeness is improving and some methods have been devised tomake use of the less than complete data. However, registration data on migrants/migrationsin most countries cannot be relied on to produce reliable estimates ofimmigrants, let alone of internal migrants/migrations. In addition, for variousreasons (illegal status, temporary residence of recent migrants, fear ofxenophobia, etc.) migrants (especially immigrants) are usually underrepresentedin censuses and surveys.
Methodsfor measuring migration are broadly similar for both internal migration (in- orout-migration) and international migration (immigration or emigration), exceptin one very important respect. A census or survey can measure internationalimmigration by identifying persons born abroad, but it is much harder toidentify emigrants because it is not possible to carry out a census/survey inall recipient countries. Approaches to estimating emigration include: (i)systematic identification of nationals in censuses of other countries (UN PopulationDivision 2011); (ii)including census/survey questions about usual household members living abroad(e.g. in the Swaziland Censuses of 1986 and 1996); (iii) asking about theresidence abroad of close relatives, especially a woman’s children or arespondent’s siblings (Zaba 1985); and (iv)using intercensal residual methods to estimate numbers of missing residents atthe time of a second census. The first approach is dependent on receivingcountries having, and being willing to share, relevant data and only capturesmigration of the native-born population; the second approach depends on the,perhaps vague, concept of household membership, and will also fail to coverentire households that have moved away; the third also fails to capture entiremissing families, does not provide estimates of recent emigration, and in smallexperimental surveys has not proven convincing. Only the fourth can be expectedto give plausible estimates of recent outflows, provided both censuses countthe population reasonably accurately, but gives no potentially usefulinformation about destination.
With these limitations and problems of accurate datacollection, the field of migration analysis has developed largely independentlyfrom mainstream demography, leading to it concentrating primarily on developedcountries where the quality of data available to measure migration is typicallymuch better than it is in developing countries, and possibly because migrationin these countries is often a matter of greater political and public policyconcern. A further consequence of these factors is that the field has developedits own terminology and techniques, which are often quite far removed from thedemography discussed elsewhere in this manual.
Definitions
Asnoted above, a migration is defined as a move across a geographically-defined(usually administrative) boundary of interest to the analyst with the effect ofchanging a person’s place of usual residence. Assuming that the boundary can beclearly defined, this immediately raises two questions: how does one defineusual place of residence, and how does one determine whether it has changed? Unfortunately,no very precise answers can be given to these two questions, giving rise to inevitableuncertainty in measurement. The preferred definition of usual residence is interms of length of residence: that if one intends to live, or after one haslived, in a place for a period of time (e.g. one year) one becomes a usualresident. Note that usual residence is not the same thing as legal residence. ThePrinciples and Recommendations for Population andHousing Censuses (UN StatisticsDivision 2008: 102, para. 1.463)defines usual residence as follows:
“It is recommended that countries apply a thresholdof 12 months when considering place of usual residence according to one of thefollowing two criteria:
(a) The place at which the person has livedcontinuously for most of the last 12 months (that is, for at least six monthsand one day), not including temporary absences for holidays or workassignments, or intends to live for at least six months;
(b) The place at which the personhas lived continuously for at least the last 12 months, not including temporaryabsences for holidays or work assignments, or intends to live for at least 12months.”
However,this definition does not deal with the situation of a person with two homes whor*gularly spends about six months in each. In general, we have to rely onpeople to self-define as residents or not, although some tests could be implemented(such as asking where their car is registered, where taxes are paid, where theyvoted, where the person sleeps at night on a regular basis, etc.). For mostpurposes, a person can distinguish between whether he or she is a usualresident and visitor, and this simple distinction suffices.
Data sources
Migrationhas been the Cinderella of demography, kept in the background as far aspossible, and dedicated migration surveys are few, far between, and specialized(an excellent example is the description of the Mexican Migration Project by Massey, Alarcon, Durand et al.(1987)).Dedicated migration surveys typically include full migration histories, which,though raising complex analytical issues, tend not to be focussed on theestimation of numbers of migrants/migrations. In this section we do not coverthe analysis of such full histories (there are very few general principles thatwould apply to a useful number), but rather deal with the sorts of datacollected by population censuses and general household surveys and sometimes,developed countries, by some form of registration.
Birthplace
Themost widely collected data relevant to migration is place of birth. Incomparison with place of residence at the time of a survey, this informationdescribes lifetime migration. The information provides limited informationabout timing of migration, and is ‘net’ migration in the sense that it misses,entirely, migrations that have been reversed (back to the place of birth) andall intermediate migrations. At the time of data collection, decisions have tobe taken about the granularity of the data: i.e., for those born abroad, howmany countries should be explicitly recorded and for those born in the country,what level of geography should be recorded. For the analyst, of course, thesedecisions were made at the questionnaire design stage, but some degree ofgreater aggregation may be required. The analysis of data on birthplace isdescribed below, but it is useful to make two points here. First, if data onbirthplace by age and sex are available for two points in time, it is possibleto estimate net migration (by age and sex) during the interval. Second,although birthplace reflects lifetime migration, the length of “lifetime”varies by age, and (provided the census data on children is reasonablyaccurate, which it often isn’t in many developing countries) the migration of0-4 year olds may be used as an indicator for recent migration of their parents(Raymer and Rogers 2007).
Residence at some specified time inthe past
Thisinformation is very often collected in addition to that on birthplace, with theexpress objective of providing data on recent migration. The time pointspecified is generally five years earlier, but sometimes a one year period isused. However, it tends to work better if the time point is associated with amemorable event, such as the previous census, on the assumption that thecoverage of that previous census was largely complete (so that people rememberbeing counted). The longer time period identifies more migrants, but missesintermediate moves, whereas the shorter time period is more susceptible toreference period error (I moved “about a year ago”).
Place of previous residence
Thisinformation is almost always collected as an alternative to residence at somespecified time in the past, and is generally combined with an additionalquestion about duration of current residence (or date of last move). Theobjective again is to provide data on recent migration.
Duration of current residence
Thequestion refers to duration of residence in the civil division (such as a townor province), not in an individual dwelling unit. This question is of limiteduse on its own and tends to be paired with the one above to provide a timeframe for estimates.
Intercensal population change
Thoughnot involving a direct question about migration, intercensal population changeby age and sex can, provided both censuses are reasonably accurate counts ofthe population, provide residual estimates of net migration between the twocensuses (Hill 1987; Hill and Wong 2005; UN PopulationDivision 1967). Intercensalpopulation change (for cohorts or age groups) by age and sex is adjusted forthe effects of intercensal fertility and mortality to provide a residualestimate of intercensal net migration (i.e., treating migration as thebalancing item in the fundamental demographic balance equation). Migration isgenerally concentrated in the age range 20 to 40, ages at which mortality ratesare, at least in the absence of HIV/AIDS, relatively low and fertilityirrelevant, so residual migration estimates are insensitive to assumptionsabout fertility and mortality (except in populations severely affected byHIV/AIDS where using these data to estimate migration is not recommended). Suchestimates are extremely sensitive, however, to even small changes in censuscoverage; such errors may be manifest in high age-specific migration rates overage 50, where migration is usually low.
Migration measures
It is not the purpose of this introduction to providea comprehensive summary of all the measures and definitions – the interestedreader is referred to the UN manual on internal migration (UN Population Division 1970) – buttwo are of particular importance for the chapters that follow.
Migration stocks
Stocks of migrants are typically thought of as numbersof persons (by age group and sex) not born in the civil division ofenumeration. The proportions born elsewhere (in the country or in othercountries) give a good general sense of the magnitude of in-migration andimmigration, but no sense of any dynamic changes that may have occurredrecently. However, changes in stocks can be used to estimate immigration (netof any onward or return migration of the foreign-born).
Migration rates
Assuming that migration events can be fully andaccurately identified, occurrence/exposure rates can be calculated forout-migration or emigration in exactly the same way as for mortality, dividingevents in a period by exposure time; such rates can be crude (both sexes, allages) or age-sex specific. The same is not the case (or at least not usefully)for in-migration or immigration, since the population exposed to the risk ofmigrating into a civil division is the entire population of the world livingelsewhere. In-migration and immigration rates are always calculated by dividingevents by the exposure time of the one population group not exposed to risk,the current residents; such rates can be crude (both sexes, all ages) orage-sex specific. Defining rates in this way has the advantage of satisfyingthe needs of the demographic balancing equation, since rates of gain and lossare measured relative to the same population. This confers a further advantage inthat net migration rates can be estimated from the demographic balancingequation as population change between two time points (e.g. censuses) minusgains due to births in the interval plus losses due to deaths in the interval. However,this approach does have the disadvantage of removing the scale limits on“normal” occurrence/exposure rates; for example, at the extreme, a person movinginto a previously unoccupied civil division creates an in-migration rate ofinfinity.
Description of methods covered
The chapters in this section focus on the estimationand quantitative description of immigration and internal in- and out-migration.They are not meant to provide comprehensive coverage of all measures ofmigration, and specifically they do not cover the important, but problematic,issue of measuring emigration (other than by mentioning that the method ofestimating immigration (net of return/onward migration) of foreigners, can beapplied to the data of the main countries of destination of emigrants to getsome sense of the age profile and magnitude of emigration.
Chapter 35 concentrates on the basic methods of usingdata from censuses to estimate the numbers (net of return/onward migration) ofimmigrants from the change in stock of foreigners, and of internal in- andout-migration from the change in stock by place of birth and from the place ofresidence at some date prior to the census.
Chapter 36 describes the selection and fitting of oneof the Rogers-Castro multi-exponential models to estimates of migrationprobabilities (or rates) derived from estimates of the number ofmigrants/migrations using non-linear optimisation procedures.
Chapter 37 describes the multiplicative and log-linearmodels for capturing, comparing and analysing the mass of inter-regionalmigration flows from places of origin to places of destination. The chapteralso provides an introduction to the method of offsets for extending the use ofthese models to estimate inter-regional flows from marginal flows (i.e. totalflows out of, or into, regions). The intention is to expand the material on themethod of offsets into an additional chapter at a later date, which will beplaced on the Tools for Demographic Estimation website.
Further reading and references
Asmentioned above, UN Manual VI (UN PopulationDivision 1970) provides acomprehensive, if dated, introduction to the description and measurement ofinternal migration. Those looking for an overview of indirect methods ofestimating migration are referred to the useful, if also somewhat dated, reviewby Zaba (1987). Morespecifically, Hill (1987) attemptedto apply the logic underlying the Generalized Growth Balance method of adultmortality estimation (described in Chapter 24) to estimate undocumentedmigration, while Hill and Queiroz (2010)sought to estimate net migration in parallel with the estimation of mortality. Unfortunatelyneither method has proved to be particularly successful.
Thoseinterested in reading more about the models of migration (multi-exponential,multiplicative and log-linear) or the method of offsets are referred to work byRogers, Willekens and colleagues (e.g. Little and Rogers (2007),Raymer and Rogers (2007),Rogers (1980, 1986) and Willekens (1999)).
HillK. 1987. "New approaches to the estimation of migration flows from censusand administrative data sources", International MigrationReview 21(4):1279-1303.http://dx.doi.org/10.2307/2546515
Hill K and B Queiroz. 2010. "Adjusting the general growth balancemethod for migration", Revista Brasileira deEstudos de População 27(1):7-20.doi: http://dx.doi.org/10.1590/S0102-30982010000100002
Hill K and R Wong. 2005. "Mexico–US migration: Views from bothsides of the border", Population and DevelopmentReview 31(1):1-18.doi: http://dx.doi.org/10.1111/j.1728-4457.2005.00050.x
Little JS and A Rogers. 2007. "What can the age composition of apopulation tell us about the age composition of its out-migrants?", Population, Space and Place 13(1):23-19.doi: http://dx.doi.org/10.1002/psp.440
Massey DS, R Alarcon, J Durand and H Gonzalez. 1987. Return to Aztlan: The Social Process of International Migration fromWestern Mexico. Berkeley and Los Angeles: University of CaliforniaPress.
Raymer J and A Rogers. 2007. "Using age and spacial flowstructures in the indirect estimation of migration streams", Demography 44(2):199–223.doi: http://dx.doi.org/10.1353/dem.2007.0016
Rogers A. 1980. "Introduction to multistate mathematicaldemography", Environment and Planning A 12:489-498. doi: http://dx.doi.org/10.1068/a120489
Rogers A. 1986. "Parameterized multistate population dynamics andprojections", Journal of the AmericanStatistical Association 81(393):48-61.doi: http://dx.doi.org/10.1080/01621459.1986.10478237
UN Population Division. 1967. Manual IV: Methods forEstimating Basic Demographic Measures from Incomplete Data. NewYork: United Nations, Department of Economic and Social Affairs, ST/SOA/SeriesA/42. http://www.un.org/esa/population/techcoop/DemEst/manual4/manual4.html
UN Population Division. 1970. Manual VI: Methods ofMeasuring Internal Migration. New York: United Nations, Departmentof Economic and Social Affairs, ST/SOA/Series A/47. http://www.un.org/esa/population/techcoop/IntMig/manual6/manual6.html
UN Population Division. 2011. International MigrationReport 2009: A Global Assessment. New York: United Nations,Department of Economic and Social Affairs, ST/ESA/Series A/316. http://www.un.org/esa/population/publications/migration/WorldMigrationReport2009.pdf
UN Statistics Division. 2008. Principles andRecommendations for Population and Housing Censuses v.2. New York:United Nations, Department of Economic and Social Affairs,ST/ESA/STAT/SER.M/67/Rev2. http://unstats.un.org/unsd/publication/SeriesM/Seriesm_67rev2e.pdf
Willekens FJ. 1999. "Modeling approaches to the indirectestimation of migration flows: From entropy to EM", MathematicalPopulation Studies 7:239-278. doi:http://dx.doi.org/10.1080/08898489909525459
Zaba B. 1985. Measurement of EmigrationUsing Indirect Techniques: Manual for the Collection and Analysis of Data onResidence of Relatives. Liège: Belgium: Ordina Editions.
Zaba B. 1987. "The indirect estimation of migration: A criticalreview", International Migration Review 21(4):1395–1445. doi: http://dx.doi.org/10.2307/2546519
