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    Coordinated Migration
    ‏By Aude Hofleitner, Ta Virot Chiraphadhanakul and Bogdan State on the Facebook Data Science Team.
    Large-scale migrations are an important part of human history. Whether they result from economics, wars, natural disaster, cultural imperatives or state policies, the worldwide flows of human beings are highly complex and notoriously difficult to measure. Here we turn our attention to a specific, fascinating kind of human migration, which we call “coordinated migration".
    Coordinated migration: 
    In a coordinated migration, a significant proportion of the population of a city has migrated, as a group, to a different city. More specifically, a flow of population from city A (hometown) to another city B (current city) is considered a coordinated migration if, among the cities in which people from hometown A currently live, city B is the city with the largest number of individuals with current city B, and hometown A.
    To study between-city coordinated migration, we examined aggregate, anonymized data on all users who list both their hometown and their current city on their Facebook profile.
    For a given hometown h, let c(h) denote the most likely current city of the users from hometown h, that is, the city c(h) is the most often listed as current city among the users from hometown h. Let p(h) denote the percentage of users from hometown h who currently live in the city c(h). Note that p(h) essentially represents the (empirical) conditional probability that a user lists c(h) as current city given that he or she lists h as hometown.
    For instance, let’s say 1,000,000 people list Boston as their hometown on Facebook. Out of these individuals, 300,000 list Boston as their current city, and no other city has more individuals listing Boston as their hometown. From the definitions above, it follows that Boston is the most likely current city for people who grew up in Boston. People who grew up in Boston still live there with a 30% probability. This is quite a common occurrence – for many cities, people are most likely to stay where they grew up. The study of coordinated migration focuses on cities for which the most likely current city is different from the hometown. For example, 67% of the individuals with Badagri, Nigeria as hometown have Lagos, Nigeria as their current city. Lagos is the most likely current city for people from Badagri.
    We formally define a coordinated migration as the movement of population from a hometown h to its most likely current city c(h) such that:
    At least 100 users have listed city h as their hometown.At least 20% of the users with hometown h have current city c(h) in their profile (p(h) > 0.2).The hometown h and its most likely current city c(h) are different cities. To reiterate, what we look into here are those cases where at least 20% of the users from city h moved out AND currently live in city c(h). For this reason, we gave the name of "coordinated" migration from hometown/origin h to current city/destination c(h).

    Note the following properties:
    Because there is a unique most likely current city for a given hometown, each city can only be the origin of a single coordinated migration.A city c can be the destination of several coordinated migrations. For such a city c, we count the number of coordinated migration flows which have c as destination and denote it n(c). For example, if there are three cities such that their corresponding most likely current city is c (with probability 20% of higher), then n(c) = 3.
    We display the coordinated migrations as follows:
    The origin cities of coordinated migrations are represented by blue dots.The destination cities of coordinated migrations are represented by red dots. For each destination city c, the size of the red dot depends on n(c), the number of coordinated migrations which end in the city, as defined above.The coordinated migrations are represented by an arc between the origin and the destination cities.
    The following map represents coordinated migrations over the world. Besides the coordinated migration, the map highlights (yellow shade) the countries with the largest urbanization growth between 2000 and 2012, according to the data from the World Bank 

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    • 2013-12-28 17:33:34Z
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