Interplay between Telecommunications and Face-to- Face Interactions: A Study Using Mobile Phone Data Francesco Calabrese1,2*, Zbigniew Smoreda3, Vincent D. Blondel4,5, Carlo Ratti2 1 IBM Research, Dublin, Ireland, 2 Senseable City Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America, 3 Sociology and Economics of Networks and Services Department, Orange Labs, Issy-les-Moulineaux, France, 4 LIDS, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America, 5 Department of Mathematical Engineering, Universite´ Catholique de Louvain, Louvain-la-Neuve, Belgium Abstract In this study we analyze one year of anonymized telecommunications data for over one million customers from a large European cellphone operator, and we investigate the relationship between people’s calls and their physical location. We discover that more than 90% of users who have called each other have also shared the same space (cell tower), even if they live far apart. Moreover, we find that close to 70% 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 indicative of coordination calls, which occur just before face-to-face meetings. Their number is highly predictable based on the amount of calls between two users and the distance between their home locations - suggesting a new way to quantify the interplay between telecommunications and face-to-face interactions. Citation: Calabrese F, Smoreda Z, Blondel VD, Ratti C (2011) Interplay between Telecommunications and Face-to-Face Interactions: A Study Using Mobile Phone Data. PLoS ONE 6(7): e20814. doi:10.1371/journal.pone.0020814 Editor: Enrico Scalas, Universita’ del Piemonte Orientale, Italy Received January 18, 2011; Accepted May 11, 2011; Published July 13, 2011 Copyright: 2011 Calabrese et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors were partially funded by the AT&T Foundation, the National Science Foundation (grant number 0735956), the MIT SMART program, GE, Audi Volkswagen, SNCF and ENEL. Francesco Calabrese was partially funded by IBM Research, which had no role in study design, data collection, data analysis, decision to publish, or preparation of the manuscript. Zbigniew Smoreda was funded by Orange Labs, which contributed to data collection and had no role in study design, data analysis, decision to publish, or preparation of the manuscript. The other funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: Francesco Calabrese is employed by IBM Research. Zbigniew Smoreda is employed by Orange Labs. These affiliations do not alter the authors’ adherence to all PLoS ONE policies on the sharing of data and materials. * E-mail: fcalabre@ie.ibm.com Introduction The interplay between telecommunications, travel and face-to- face meetings is an unresolved puzzle. In some cases it has been suggested that telecommunications may be a substitute for physical interaction [1] - an idea that gained traction during the nineties and the rapid expansion of the Internet [2,3]. In other cases conflicting hypotheses have been made, including those of a complementary [4,5], neutral [6] or reinforcing [7] effect. Recently, social networks have been identified as possible predictors of travel behavior, as well as the possible decision to telecommute [8,9]. Social interaction has thus been integrated in activity-travel models [10], in addition to the existing categories of travel such as commuting, leisure and business. Furthermore, researchers such as Urry and others [11–13] have argued that flows and meetings of people produce small worlds, which require connections and meeting places - a phenomenon which is also known as the new mobilities paradigm. This study aims to provide a new perspective into the relationship between telecommunicating people and their physical locations through an assesment of anonymized Call Detail Records (CDRs). CDRs show great promise for academic research: they have recently been used to explore human communications [14,15], the geography of social networks [16,17], urban dynamics [18], and human mobility patterns [19–22]. In this paper we use them for the first time to study the relationship between the telecommunications patterns of any two people and their physical locations. Results 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 (see Methods). We look at all communications between pairs of users, together with their locations at call time. As we are interested in comparing people locations, we discard users for which we do not have enough samples. We use two subsets: D1, which contains all reciprocal communications between the top 100,000 callers; and D2, which contains 10,000 pairs from D1, sampled at different home distances to ensure the same home distances distribution found in D1 (see Text S2). In the sequel, we use D2 in cases where computational complexity limits the use of a larger set. We discover that at least 93% of users in D1 who reciprocally call each other, have at least once shared the same cell tower area in one year. The percentage decreases slightly as the distances between their homes decreases, but the value is still above 90% for users living 100 km apart (see Figure 1). It appears that almost all remote communications are associated with being physically sharing space. It may also be noted that we are underestimating the percentage as our data is only based on locations at call time, so users might have also shared space without this being recorded in our data. Results are consistent with what was recently found analyzing spatio-temporal coincidences in a geo-tagged pictures database to infer social ties [23]. If we also consider the temporal component, we can look at how often and where users are sharing the same space at the same time. PLoS ONE | www.plosone.org 1 July 2011 | Volume 6 | Issue 7 | e20814