44 Chapter 3 Offices tend to assemble data from population registers that was originally gathered at the municipal level and various historical and administrative contexts are then reflected in the way countries choose to collect and record their migration data. Essentially, national data sources tend to reflect national regulatory frameworks in the respective reporting country (Lemaitre, 2005). Plus, there is little incentive to change because the data that is collected and the methods used are usually considered adequate for the administrative, fiscal, and planning needs of the local authority. Consequently, harmonising population register criteria according to international guidelines has been exceedingly difficult (Kraler, Reichel, & Entzinger, 2015; Lemaitre, 2005). 3.4.2 Stock versus flow measures of migration The most reliable data sources tend to be on stocks of migrants, e.g. foreignborn as a percentage of the population, rather than flows of immigration. Most EU countries identify the foreign-born population in their censuses making it possible to compare the size of migrant populations (Lemaitre, 2005). However, even for an indicator as simple as the foreign-born population there are still problems regarding definitions and measurement methods. For example, persons who are nationals of their current country of residence but were born abroad are included in the foreign-born data. Moreover, some countries – such as Germany – do not always collect data on foreign-born residents but rather base national statistics on their foreignpopulation (individuals with a foreign passport) (de Beer et al., 2010; Laczko, 2015; Lemaitre, 2005). This means that once a person has naturalised in their host country, they are no longer considered an immigrant. This can change the size of the reported stock and introduce bias if certain groups with certain characteristics are more likely to naturalise than others. Data on flows of migrants are more problematic as measurement is difficult ensuring that many countries do not possess reliable statistics. For emigration in particular, outflows of people tend to be unregulated or unchecked and there is little incentive for a migrant to report their movements to the country they have left, thus it is not uncommon for emigrants to still be registered in their country of origin even after living abroad for several years (de Beer et al., 2010). As a result the numbers of emigrants reported by a sending country often differ from the corresponding receiving country’s reported number of immigrants (DeWaard et al., 2012). Moreover, the recording of national immigration (individuals holding a passport of the country they are immigrating to) is often supressed by the receiving country (de Beer et al., 2010). Consequently, immigration, emigration, and net migration are measures that are subject to a degree of uncertainty. These difficulties are further compounded in an area of free movement as there are no border checks, no need for to apply for a work or residence permits, and no incentive to register one’s movements. Hence, data on intra-EU
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