Telecommunications and Face-to-Face Interactions Figure 1. Probability that two reciprocal calling users have shared the same cell tower area during a 1 year time (D1 subset). doi:10.1371/journal.pone.0020814.g001 We restrict our attention to the case when two users call each other using the same cell tower. This scenario is based on the hypothesis that they are calling each other to coordinate to meet in a nearby area, also called ‘‘coordination knot’’ [24]. Of course, two people living or working close by could also call each other very often without physically meeting. So, we excluded users living or working in the same cell tower area, estimated as described in Text S2. We define a co-location event between two users (who live and work in distinct locations) as a call between the users while they are connected to the same cellphone tower. Each co-location is characterized by a specific time and place. Based on this definition, we characterize the spatio-temporal features of co-location events, to see whether they represent a reasonable subset of actual face-to-face meetings between users. Starting with the larger subset (D1), we analyze the relationship between calling activity and user’s locations. Among the pairs of communicating users, 400,000 cases have two users calling each other while in the same cell tower area, 350,000 of which have distinct home and work locations. Interestingly, 38.33% of the communicat- ing users co-locate at least once during the period examined. When stronger relationships are considered (users who call on average at least once per month) the percentage increases to 69.41%. Call duration appears to increase with the homes distance between users (see black line in Figure 2). Calls that occur between co-located people (red line) have a shorter average call duration, suggesting that people who co-locate call each other briefly to coordinate the exact meeting place and time. We also find that the number of calls between two users increases just before and after their co-location (Figure 3). The probability is rather constant in the interval, with two peaks around 0 and 1 (consecutive co-location events). The presence of these peaks suggests that the considered events (co-locations) represent a reasonable proxy for face-to-face meetings. In particular, a peak of calls just before the co-location event, suggests that the two people are talking on the phone to arrange a meeting, in line with what is hypothesized in [16,24]. The peak PLoS ONE | www.plosone.org 2 July 2011 | Volume 6 | Issue 7 | e20814 right after the co-location event might be explained by a follow up call after the meeting. We analyze the features of co-location places and compared it with geographical and communication differences between users. We define d1(l) and d2(l) as the distances traveled by two users at every co-location event l~1, . . . ,m, and compute three measures of comparison: 1. The median ratio between the shortest and longest distance at co-location time: rd ~medianl minfd1(l),d2(l)g : maxfd1(l),d2(l)g 2. The fraction of times user 1 travels less than its peer: where: rt1~1=m m X g(d2(l){d1(l)), l~1 g(x)~ 1 if xw0 0 if x~0 : rt~minfrt1,rt2g: 3. The fraction of times one of the users travels less than the peer: The first measure rd allows a comparison to be made between the lengths of the two users’ trips. On the D2 subset, we find on