Migration is not easy to measure. It can be classified by several factors including

  • administrative border crossing (international or internal);
  • direction (inward, outward, origin-destination);
  • purpose (labour, education, social, economic, ecological, political, conflicts, etc.);
  • duration of stay.

Definition of internal migration usually varies across the countries, i.e. relies on the definition of the usual residence and data source(s). The United Nations Economic Commission for Europe (UNECE) for the census purposes defines internal migrants as "persons who are usually resident in a particular geographical area and who have previously been resident in another geographical area in the country of enumeration".

By providing a clearer concept, the UN is addressing the confusion between international migration and other types of international mobility so that countries may provide more reliable and comparable statistics. According to UN recommendations, international mobility includes all movements that cross international borders within a given year. International migration is a subset of international mobility and includes all movements resulting in a change in the country of residence within a given year".
According to UN recommendations an international migrant is defined as a person who has changed his or her country of residence and established new residence in the country within a given year. An international migrant can be either 'immigrant' or 'emigrant' and include those with national or foreign citizenship or stateless persons.

The resident population consists of individuals who either (a) have lived most of the last 12 months within a given year or have intentions to stay (or granted to stay) for at least six months; or (b) have lived at least 12 months within a given year or intentions to stay (or granted to stay) for at least 12 months, not including temporary absence for holidays or work assignments. A person's country of usual residence is that in which the person lives, that is to say, the country in which the person has a place to live where he or she normally spends the daily period of rest.

International temporary mobility refers to all movements that cross the international border that does not result in a change in the country of residence.

Cross border workers include all persons who are not the residents of the country of measurement but have been engaged in economic activities on a repeated basis in that country provided they depart at regular and short intervals (daily or weekly) from the country.

Seasonal workers include all persons who are not the residents of the country of employment, whose work by its character is dependent on seasonal conditions and is performed during part of the year.

Seasonal mobility is characterized by changes in place of stay which last for more than one and less than twelve months and are repeated on a similar basis each year. Seasonal migration depends on factors that are geographically related both to the starting point and to the destination of the migration, as well as the motives. In the case of migration related to seasonal work, for example in agricultural regions, peak population numbers occur during the harvest period. Cold climates and coastal regions have more temporary residents in summer than in winter. Areas with warm climates have more temporary residents in winter. Mountainous regions, especially ski resorts, often have both summer and winter seasons. Seasonal changes in social behaviour depend on a wide range of circumstances:

  • Structural factors connected with the impact of the external environment, e.g. natural factors (changes in weather or length of day) or social structure (changes in timing of holidays, traditions, and outdoor activities);
  • Internal (actor-related) factors – processes taking place within persons, e.g. biological clock ", as a programmed reaction to factors from the external environment. It is worth to mention that a few studies have been carried out regarding how seasonality influences human behaviour (Silm & Ahas, 2010).

he key concepts and definitions described above are mainly applicable for traditional data sources. Applying new technologies for the human mobility analysis based on the classical definitions seems unrealistic and causes much uncertainty. In order to identify a migrant, detailed algorithm of the data processing and methodology has to be developed. It has to be noted that compliance with the UN principles and recommendations and considering the best practices are also very important.

The main methodological challenges in using MPD to measure migration are identifying the place of residence (home), and the identification of the time period when a certain place is to be considered home and when it is not (has changed).

Likewise, the MNOs do not cover the whole population and there is a need for some statistical methods and skills, e.g. with extrapolation methods, the absolute numbers could be presented. But if there is no reliable reference data to calibrate MPD results, relative numbers regarding migration are more reliable (e.g. the number of subscribers with changed home location compared to the total number of home locations in an area). Extrapolation to the general population is problematic, as the number of mobile devices or Subscriber Identity Module cards (SIM cards) is never equal to the number of people.

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