From the point of view of MPD, the concept of migration can be defined as the international mobility of subscribers. Several factors need to be taken into account.

Basic assumptions. If a person lives and is active in several spaces, his or her phone number(s) will be registered in one or several of these countries. Usually, a mobile operator is chosen from the country that the person has the strongest connection to, where they spend more time than elsewhere and where the important people and social networks are located. It may also be important to be available for business, social or state service reasons. If a person decides to commit permanently to a destination and switches off the mobile phone number (SIM-card) of the previous homeland, this is a sign of being estranged from the previous homeland. It should be noted that roaming rates matter as well.

The problem arising when analysing international migration is that local MNOs might not possess information on the person's movements abroad unless he/she is not using roaming. However, border cross data (dates of inward/outward crosses) can help to infer missing information about the SIM card activity (person's). Based on that information, the duration of being abroad can be assessed.

Some assumption-based methods might also be applied. For example, the disappearance of foreign or the appearance of new domestic SIM cards can suggest the arrival of migrants in specific areas of the country.

Steps and algorithms used for identifying international migrants for the year X include:

Step 1. Identify the date of the first cross (movement from home country to abroad) of a person in X year (year, day, month);

Step 2. Sum up the number of nights a person spent at home country during the 12 months before the first cross, and identify those persons who have cumulated at least 183 days;

Step 3. Sum up the nights a person spent abroad during the 12 months after the first cross, and identify those persons who have cumulated at least 183 days



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