A woman takes pictures with a mobile phone from behind a barrier separating women from men at Taghyeer (Change) Square, in Sanaa, January 26, 2012.
A woman takes pictures with a mobile phone from behind a barrier separating women from men at Taghyeer (Change) Square, in Sanaa, January 26, 2012.
Khaled Abdullah / Reuters

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.

Yemen Cell Phone Usage
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.

Yemen Cell Phone Usage Weekdays vs Fridays
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 chew session, which lasts for three to four hours after lunch. It is then followed with a spike in calls between 6:00 PM and 9:00 PM. The Yemeni weekend begins on Friday, with a big deviation from the weekday call patterns right after noon, during Friday prayer (salaat al jumah). This is the most well-attended session of the week and involves a sermon by a religious leader followed by a communal prayer.

Yemen Cell Phone Usage Ramadan
Cell phone usage captures not just everyday behaviors, but also those during religious holidays, such as the holy month of Ramadan. Ramadan is a period of purification and self-reflection for Muslims the world over, and observant Yemenis abstain from all food and drink from sunrise to sunset. The fast is broken every day at dusk with meals in family or communal gatherings. This evening meal (iftar) is followed by socializing until the early morning meal (suhoor) before the predawn prayer, which signals a new day of fasting. Working hours are truncated during Ramadan, and people tend to stay up at night and sleep during the day. This flipping of day and night activities is reflected in the mirror image of call activity, with a notable dip in calls during the iftar meal at sundown followed by a subsequent increase in nighttime calling

Yemen Cell Phone Usage Drone Strike
Yemen has experienced an array of violent incidents and political turmoil in recent years, ranging from al Qaeda militant attacks to drone strikes, Arab Spring protests, and now Saudi Arabian air strikes. Call patterns can capture political or violent activities as they unravel in real time. For instance, there was a significant increase in the number of local calls following a nighttime drone strike on Friday, October 14, 2011, in Shabwa Province, which killed Ibrahim al-Banna, media chief of al Qaeda in the Arabian Peninsula, along with eight others. Several additional drone strikes are detectable through significant spikes in local call volume at the time of attack, including a May 5, 2011, strike in Jahwa that killed two brothers affiliated with al Qaeda; a May 10, 2012, attack that killed eight al Qaeda militants in Jaar; and a May 25, 2010, attack in Wadi Abida that killed two militants and four to six civilians.

Yemen Cell Phone Usage Bombing of Presidential Palace
Because call data records are geo-located, researchers can also examine them to assess users’ mobility. This can be a very useful tool to detect protests and other events that entail notable mobilization. On Friday, June 3, 2011, for example, the day President Ali Abdullah Saleh was attacked at the presidential palace in connection with the Yemeni Arab Spring, the number of people near the palace spiked around the time of the incident. Several other protest events linked to the Yemeni Arab Spring are captured by high levels of user mobility, including the string of Friday protests during 2011 when protesters would gather at Sanaa University Square and government supporters would stage their counterrallies at Sanaa’s Tahrir Square.

Yemen Cell Phone Usage Saudi Arabia and Iran
Call records from recent years also reflect Yemen’s relations with its important neighbors, Saudi Arabia and Iran. Ties with the Saudi kingdom are robust and long-standing. Beyond sharing a border and a language, an estimated one million Yemenis live as migrant workers in Saudi Arabia and send home approximately $1.4 billion in remittances annually. No such connections exist with Iran. It is therefore not surprising that, as indicated in the bar graph, the number of calls between Yemen and Saudi Arabia, and the number of individuals making such calls, are many orders of magnitude higher than the corresponding figures associated with calls between Yemen and Iran.

Almost three months since the start of the Saudi-led bombing campaign in Yemen, the Houthis—a Shiite group that expanded its influence from the northern province of Saada this September into large swaths of the country and drove President Abdu Rabbu Mansour Hadi to exile in Saudi Arabia—claim to be fighting against a corrupt government and the rise of Sunni Muslim extremism in Yemen. Meanwhile, the Saudi-backed Yemeni government of Hadi portray the Houthis as polarizing agents of Iran and demand their withdrawal from the large Yemeni cities they control.

Call records from the northern Houthi stronghold of Saada over three years predating the Houthi expansion across Yemen don’t show any particularly strong connections to Iran, with fewer than 500 calls to and from Iran in that period as compared with almost five million calls to and from Saudi Arabia.

Yemen Cell Phone Towers Iran
Indeed, as indicated in the heat map, most calls to and from Iran happen in Yemen’s largest cities of Sanaa, Taiz, and Aden and are dwarfed by communications with Saudi Arabia.

Yemen Cell Phone Towers Saudi Arabia
These graphs and maps show that religion is central to Yemenis’ everyday lives and that khat rituals have a notable effect on many people. They also suggest that Yemenis care about politics, given their massive mobilization during the Arab Spring, and that they react to drone strikes in real time. Last, despite Iran's alleged involvement in the current conflict, call data records show that Saudi Arabia remains a considerably more integral player in Yemeni lives.

With such high-resolution data, the opportunities to improve social, economic, and even military policy are numerous. Knowing the timing, geography, and intensity of khat chews, for example, offers an empirical basis for interested policymakers to begin curtailing this economically burdensome practice. The same data provide insight into the local impact of drone strikes, including the paths and extent that word of an attack travels.

Reliable information on sensitive issues such as these would be difficult to collect using surveys and media reports in even a permissive political environment. In a place such as Yemen, where there is poverty and war, the challenge is all the greater. With the aid of big data, and cell phone records in particular, we now have the ability to overcome these informational barriers and vastly improve our understanding of the most troubled parts of the world.

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