Using cell phones to compute dynamic population densities safely

Active cell phones are used as a basis for recording the presence of cell phones in calls (at cell phone level). This basic information is used to compute various population densities (such as municipal densities) as well as densities based on smoothed Voronoi densities. Not only static densities per hour block can be computed, but also the dynamics of these densities using Markov chains. The computations are safe in the sense that the no cell phone user can be re-identified.

The method proposed is based on anonymous cell phone data. No cell phone is tracked over any length of time. To compute the dynamics, the telecom providers who own the cell phone data have to prepare traffic like data. With these data it is possible to compute how much movement there is from one cell to another cell, from one particular hour block to the next one.

Potential problems with such data may result from several kinds of biases that lurk in these data: certain age or ethnic groups that are over or under represented. The openness of the population of active cell phone users is an issue to consider, as these users may decide to be active or not on their cell phone at a particular moment. Or they may leave or enter the country, at will.

Population densities may be represented numerically (at the municipal level, for instance) or they may be plotted as basic densities such as (smoothed) Voronoi densities, derived from the Voronoi polygons generated by the locations of the cells.