Telecommunications and Face-to-Face Interactions Figure 4. Normalized number of co-locations, calls, total call duration as function of users’ home distances. doi:10.1371/journal.pone.0020814.g004 strength differs between users, the more the co-locations occur in places close to one of them. Our definition of distance d is based on the Euclidean distance between home and co-location places. Two limitations arises from this choice: 1) the Euclidean distance does not take into account the real path taken by a person; 2) the person might not travel directly from home but the origin of the trip to the co-location place could be different. However, as we are interested in the relative distances traveled by the two peers, we can assume that both limitations affect the two measures in a similar manner, thus limiting the potential bias. We evaluate the relationship between the home locations’ distance and the number of co-locations between users. Figure 4(a) shows the average number of co-locations, which decreases with distance. The result is consistent with what was found in [12,25– 27] using data from surveys. If we compare this decrease with the one of phone calls, and total call times (see Figure 4(a)) we find different decays with distance. Total call time is the least affected by distance (slope 20.04), followed by the number of calls (slope 20.07). In contrast with this, the number of co-locations is strongly affected by distance (slope 20.14). Even if we consider a broader definition of co-location, in which two users are considered co- Figure 5. Average number of co-locations as a function of the number of calls (D1 subset). doi:10.1371/journal.pone.0020814.g005 PLoS ONE | www.plosone.org 4 July 2011 | Volume 6 | Issue 7 | e20814