How to map wealth in Africa using nothing but mobile-phone minutes

A group of Belgian researchers has come up with a novel way (pdf) of mapping the distribution of wealth in Côte d’Ivoire—by looking at purchases of mobile airtime minutes. After combing reams of data on individual purchases from a national mobile operator (unnamed in their report), they discovered that, in a country where prepaid phones dominate, there are basically two kinds of airtime buyers: those who make big infrequent purchases, filling up on minutes, and those who buy fewer minutes more regularly.

Their theory is the re-fillers are richer than those who buy airtime often, and thus the average size of an airtime purchase can serve as a proxy for wealth. And indeed, the biggest average purchases, as the map below shows, happen in Abidjan, the capital; in a few cities in the interior; and around key border areas and trade routes.