  We present a link-centric approach to study variation in the mobile phone
communication patterns of individuals. Unlike most previous research on call
detail records that focused on the variation of phone usage across individual
users, we examine how the calling and texting patterns obtained from call
detail records vary among pairs of users and how these patterns are affected by
the nature of relationships between users. To demonstrate this link-centric
perspective, we extract factors that contribute to the variation in the mobile
phone communication patterns and predict demographics-related quantities for
pairs of users. The time of day and the channel of communication (calls or
texts) are found to explain most of the variance among pairs that frequently
call each other. Furthermore, we find that this variation can be used to
predict the relationship between the pairs of users, as inferred from their age
and gender, as well as the age of the younger user in a pair. From the
classifier performance across different age and gender groups as well as the
inherent class overlap suggested by the estimate of the bounds of the Bayes
error, we gain insights into the similarity and differences of communication
patterns across different relationships.
