The Urban Village: Data Viz of How Human Interactions Scale with City Size

The white paper (http://senseable.mit.edu/urbanvillages/img/manuscript.pdf) gives the assumptions to explain why call data is representative: "Mobile phone communication data are not necessarily a direct representation of the underlying social network. For instance, two individuals may maintain a strong tie through face-to-face interactions or other means of communication, without relying on regular phone calls [23]. Nevertheless, despite such a po- tential bias, a recent comparison with a questionnaire-based survey has shown that mobile phone communication data are, in general, a reliable proxy for the strength of individual- based social interactions [25]. Moreover, even if two subscribers maintain a close relationship and usually communicate via other means, it seems reasonable to assume that both individ- uals have called each other at least once during the relatively long observation period of 15 months, thus reducing the chance of missing such relationships in our network [21, 26, 27]"

Without knowing more about the data, one possible cause might be sim cards that are not used by actual humans, but for example by smart meters, security equipment, etc., which only have to report to one central location and thus have only one contact.

When using mobile phone data, filtering out such "artifacts" can be a big issue and it might not be possible to do accurately