Estimating the numbers of undetected victims of Trafficking in Persons (TiP)
UNODC highlighted the potential of the Multiple Systems Estimation (MSE) methodology for estimating the numbers of non-detected victims of TiP in a country (UNODC, 2016). The MSE uses existing lists of victims identified by multiple government agencies, international organizations or NGOs to estimate the number of non-detected victims by way of the capture-recapture method. This method is often illustrated with the example of fish in a pond. One catches 100 fish, tags them, and throws them back into the pond. Sometime later, one catches another 100 from the same pond and counts how many of them were caught in the first round. If the overlap is close to zero, or very small, then one can assume that the population of the fish in the pond is much larger than 100 whereas if the overlap is considerable, the population of the fish is likely to be smaller. The UK’s Home Office was one of the first organizations to apply the MSE to available TiP data, demonstrating that the hidden number of victims was three to five times higher than the number of identified victims.
Estimating net international migration using census data
Data from two consecutive censuses can be used to estimate net international migration in the intercensal period (UN DESA, 2022). The net international migration refers to the number of persons who entered the country to reside there during the intercensal period minus the number who left during the same period to reside abroad. The most common method is the intercensal component equation, which demonstrates that the change in the total resident (foreign-born) population between the first and second census results from both natural increase, which is the difference between the number of births and deaths that occurred, and migration. Net international migration = foreign-born immigration (-) foreign-born emigration (+) net migration of native-born.
Estimating foreign-born and native-born emigration
The US Census Bureau provided an overview of methods for estimating emigration, one of which is the residual method (US Census Bureau, 2013). This approach involves subtracting the population count of the current census from the population count of the previous census, taking into account any deaths or new arrivals during the intercensal period. However, because emigration of the native population may be less common in certain contexts, this approach is mainly used to estimate the emigration of foreign-born populations, where the number of foreign-born emigrants = population of foreign-born from first data source (-) deaths of foreign-born (+) new arrivals of foreign-born (-) population of foreign-born from second data source.
A variant of the residual method can be applied to data from other countries to estimate the population of native-born emigrants. This is done by compiling data on the stock of native-born from the consecutive censuses, surveys or population registries of other countries. The stock of native-born counted at time 1 is survived forwards to time 2 to determine the expected population in time 2. The difference between the expected population and the actual population of native-born individuals in time 2 represents the net emigration of native-born individuals.
However, potential under-coverage of the native-born population living in other countries is likely to affect the reliability of estimates. Another method of emigration is the multiplicity sampling method. This can be conducted on the basis of a nationally representative survey including questions on immediate relatives who had previously lived in the country but were living abroad at the time of enumeration. The data is weighted to account for the probability of selection for all possible relatives for each emigrant and the probability of selection for the emigrant in the population. However, this method is not able to estimate emigrants who do not have relatives in the US.
Calculating migration rates
Migration rates capture the intensity of migration, which can be useful for comparing migration across countries/regions and studying the drivers and effects of migration. At the most general level, the migration rate is the share of migrants of a given type (e.g., from a specific time period or specific demographic group) from the overall population (Kirchberger, 2021). Some studies measure the overall level of mobility in the population by calculating the percentage of the population residing in a different locality from where they were born or where they lived one or five years prior to enumeration using census or household survey data. In the absence of information on respondents’ previous place of residence (meaning that migrants cannot be identified among the relevant population) the net overall migration rate can be calculated based on the vital statistics method. This involves calculating the difference in population size between two time points and subtracting the difference between the number of births and deaths over the same time period. This indicator can be further disaggregated by geographic region or by cohort (Kirchberger, 2021).
Estimating irregular migration
Estimating irregular migration using traditional data sources can be challenging. One methodological example comes from The Pew Research Centre (Connor and Passel, 2019). The Centre uses the American Community Survey, the Census and the Current Population Survey to estimate the total foreign-born population in the US. Subsequently, the number of people residing legally in the US is derived from demographic estimates of lawful permanent residents (LPRs). The latter figure is subtracted from the total number of foreign-born individuals, yielding the estimated number of undocumented immigrants in the country.