Telecommunications and Face-to-Face Interactions Figure 6. A prediction of the number of co-locations. Error bars represent the standard deviations (D1 subset). doi:10.1371/journal.pone.0020814.g006 located in the same cell tower if they happen to make a phone call (not necessarily to each other) from the same cell tower area within one hour, we still find a similar decreasing trend, as shown in Figure 4(b) computed for the D2 subset. The results are consistent with those from the analysis of fixed phone data combined with interviews showing the effect of distance on call duration and frequency of meetings [28,29]. The number of calls has a strong influence on the number of co- locations, suggesting that the more people call each other, the more they co-locate (see Figure 5). As there appears to be a clear relationship between call patterns, distance and co-locations, we tried to built a predictor of the number of co-locations, starting from a measure of interaction (number of calls) and the geographical distance between users’ home, obtaining r2~0:61 with the model (Figure 6): #colocations~0:92 #calls0:60 distance0:08 : This result suggests that geography and telecommunication interac- tions account for 61% of variations in the number of co-locations (see also Text S4). This is consistent during the one year time frame under analysis, as reported in Text S5. The exponent 0:60 for the #calls reveals the correlation between an increase in the number of calls and an increase in the number of co-locations. This result suggests that telecommunications might play a complementary role in facilitating face-to-face interactions, supporting the observations found in other studies [4,5]. Discussion In this study we analyze one year of telecommunications data from a large European cellphone operator to investigate the relationship between people’s calls and their physical location. We discover that more than 90% of users who called each other have also shared the same space (cell tower), even if they live far apart. Moreover, we find that 69% of users who call each other frequently (at least once per month on average) have shared the same space at the same time - an instance that we call co-location. Co-locations appear highly indicative of coordination calls occurring just before face-to-face meetings. We are able to predict 61% of variations in the number of co-locations from the number of calls, and users’ homes distance. In particular, as the distance between homes increases, the expected number of co-locations decreases. We also characterize the co-location places in terms of distance from the home locations. As the users’ homes distance increases, co-locations occur in a place that is closer to one of the users. In more than 90% of the cases, co-locations take place in an area that is closer to the same user of the pair (there is low reciprocity in the travel distance covered). Telecommunication strength helps predict which person of the pair travels less. We believe that the above results suggest new ways to use CDRs to investigate the old conundrum of the interplay between telecommunications, travel and face-to-face meetings - with applications in the social sciences, urban planning and transpor- tation studies. Methods Dataset We use a large anonymized dataset of billing records for over one million mobile phone users, which was gathered in Portugal over a twelve month period between 2006 and 2007. To safeguard personal privacy, individual phone numbers were anonymized by the operator before leaving storage facilities, and they were identified with a security ID (hash code). Each entry in the dataset has a CDR, which consists of the following information: timestamp, callers ID, callees ID, call duration, callers cell tower ID, and callee’s cell tower ID. This metadata on each call allows us to study both the mobile social interaction as well as the physical PLoS ONE | www.plosone.org 5 July 2011 | Volume 6 | Issue 7 | e20814