The proliferation of sensor-studded cellphones could lead to a wealth of data with socially useful applications such as urban planning, epidemiology, operations, research, and emergency preparedness, among other things. Of course, before being released to researchers, the data would have to be stripped of identifying information.

But how hard could it be to protect the identity of one unnamed cellphone user in a data set of hundreds of thousands or even millions? According to a paper appearing this week in Scientific Reports, harder than you might think. Researchers at MIT and the Université Catholique de Louvain, in Belgium, analyzed data on 1.5 million cellphone users in a small European country during a span of 15 months and found that just four points of reference, with fairly low spatial and temporal resolution, was enough to uniquely identify 95 percent of them.

 …