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.