Spain's National Statistics Institute (INE) is currently engaged in an experimental project using mobile phone data to identify numbers of tourists in the Balearics on any given day and the number of Balearic residents who are outside the region.
The sources of the statistics are aggregate data provided by the three large mobile phone operators in Spain and phone data for foreign tourists. The INE is receiving data in aggregate form and without any type of individual identification, which makes it impossible for it to carry out any individualised monitoring of any phone user.
There is a recognition that there are operational limitations to this. It could be, for example, that a phone is disconnected, that there is a card that can't be detected, or that someone has more than one phone.
An interest in this project is to be able to get an even more accurate picture of the whole population than at present. The peaks of human pressure on the islands occur in August, when there can be more than two million people on any given day. While these statistics take account of tourist arrival data, a different factor is how many residents are not on the islands. In May this year, for instance, the project revealed that 31,591 residents were elsewhere.