Census by Cell Phone? Maybe

A "census from heaven" is how Paul Sutton and his colleagues labeled their attempt back in 2001 to estimate global population distribution from night-time lights satellite imagery--a method that is still used, by the way. Many others have used satellite imagery in a variety of other ways to accomplish the same objective, including LandScan, Gridded Population of the World, and the U.S. Census Bureau's DemoBase. Among the currently most ambitious of the efforts to use only satellite imagery to estimate population size is the WorldPop project led by Andy Tatem at the University of Southampton in the UK, as I noted a few months ago.

So, it was with considerable interest that I saw that Tatem was among those who just published a paper in the Proceedings of the National Academy of Sciences showing how cell phone tracking might also be a viable way of estimating populations in otherwise hard-to-measure populations. The online service of the AAAS covered the story:

Ninety-six percent of the world’s people have active cellphone subscriptions. In developed countries, the number of mobile phone subscribers has surpassed the total population as some individuals own more than one phone, and subscription rates continue to rise in developing countries, reaching as high as 90%. That’s great news for census scientists, because they can locate the calls by identifying the cellphone towers that send and receive them and use call density around the phone towers to estimate the local population density. [SEE MAP BELOW]

And, yes, before you blink twice and say that can't be, the paper does provide a citation for that 96% coverage number. Indeed, in Ghana, where I and my colleagues make extensive use of satellite imagery, it is now almost unheard of not to own a mobile phone. The combination of communication satellites and cell towers has put the world into the hands of nearly all humans, and that means we can figure out where they are (or at least which cell tower they are closest to at a particular time on a given day). Will this replace the census?

The study shows the merit as well as limitation of big data, says statistician Tom Louis, chief scientist at the U.S. Census Bureau at Suitland, Maryland, who was not involved with the work. Though the information is timely, it is not yet accurate enough for official use, he says. “Big data can be very valuable, but at least at this point in our history, it needs the validation of traditional surveys to show that it works.”

But for low-income countries, where census data are likely outdated and unreliable, mobile phone records present an easy and efficient alternative, Linard says. In the Democratic Republic of the Congo, for example, the most recent census took place in 1984. In contrast, about 70% of the people subscribe to mobile phones.

Population distribution is a start, of course. Eventually we want to fill in those numbers with sociodemographic characteristics. That's where social media accessed by those mobile phones may come into play. Don't hang up...