Alters as species: Predicting personal network size from contact diaries

• Like wildlife species in an ecological system, alters in an egocentric network are often hard to count.
• We estimate personal network size based on the idea of species accumulation curves in ecology.
• A model using 48 three-month contact diaries predicts the network size in a reliable manner.
• In ecological terms, each personal network is an island, while each day of the diary becomes a trap.
• The results are cross-checked with a sensitivity analysis of incomplete data.

Like wildlife species in an ecological system, members within a personal network (or alters) constantly shift and often remain hard to count. Previous studies often estimated the size of such personal networks using information given by a focal person (or ego), who names a list of friends and acquaintances, or someone known or related, that meet certain specified criteria. In a search for alternative methods, we estimate the number of alters using contact diaries that help reveal active and comprehensive interactions, which enable us to predict personal network size from a longitudinal perspective. By exploring contact frequencies between ego and alters, we propose a modeling approach based on species accumulation curves from ecology. Under this approach, the contact frequency between ego and alter often turns out to be a mixture of binomial distributions, and the number of alters with whom ego may make contact in the future is assumed to follow a specified discrete distribution. We estimate the model with the Bayesian nonparametric method, in which the distribution of contact probabilities is assumed to be a mixture of Dirichlet processes. We then demonstrate this approach with a data set containing 48 contact diaries collected over three months and discuss how such an ecological analogy may enrich social network studies.