Telecommunications and Face-to-Face Interactions Figure 2. Average length of a call as a function of the users’ home distances (D1 subset). doi:10.1371/journal.pone.0020814.g002 average rd ~0:3, i.e. one user travels about 3 times less than the other one. Due to the asymmetric behavior in the length of trips, we question whether the shorter trips are always taken by the same user, or if the two users share the short trips. The third measure rl allows an evaluation of the asymmetry at the pair level, showing an average of 0:06. This suggests that in 94% of the selected pairs, there is one user who constantly travels less than its peers. The second measure rl1 is a directed measure and is computed to see whether geographical and communication differences allow the user that travels less to be predicted. Text S3 reports how these measures vary with homes distance, population density, normal- ized tie strength and call direction. In particular we find that as users’ homes distance increases, co-locations occur in a place that is closer to one of the users. Moreover, the more the normalized tie Figure 3. Number of calls between consecutive co-locations. Call times have been normalized to the range of 0 to 1 (D2 subset). doi:10.1371/journal.pone.0020814.g003 PLoS ONE | www.plosone.org 3 July 2011 | Volume 6 | Issue 7 | e20814