Advanced information technologies have revolutionized the way the world works and how people conceptualize it. Massive troves of information, known as “big data,” are aggregated and shared on a daily basis, recording an array of human behaviors and interactions at an unprecedented level of granularity. One form of such data is call data records, which, while preserving the anonymity of subscribers and the privacy of content, allow researchers to track the volume of traffic, timing, and location of calls. In combination with increasingly powerful computers, such data have shed light on important questions in the developed world on topics including marketing, health care, urban planning, and environmental policy.
Call data can also help us understand violent places in the developing world that are largely inaccessible. At a time when Yemen remains highly volatile, for example, anonymous Yemeni cell phone metadata from 2010 to 2013 that include over ten million users and several hundred million calls vividly capture patterns of Yemeni daily life, as well as celebrations, religious practices, involvement in politics, and reactions to violence.
Although only 20 percent of Yemenis have access to the Internet, roughly 80 percent of them—21.3 million out of an estimated population of 26.7 million—use cell phones. As indicated in the heat map, most call activity is concentrated around the highly populated urban centers of the capital, Sanaa, and the cities of Aden, Hudaydah, Mukalla, and Taiz (highest-density areas in blue). There is also more activity in the west of the country, not surprising since the east includes large stretches of unpopulated desert.
A look at cellphone usage during an average weekday clearly indicates that Yemenis start making calls soon after the first prayer of the day (salaat al-fajr). Calling activity increases in the morning hours until about lunchtime, when the vast majority of Yemenis go to the market to buy khat, a green plant known for its amphetamine-like stimulating qualities. (Estimates suggest over 90 percent of Yemeni men regularly chew khat.) The volume of calls plateaus during the daily khat