... Title: A COMPARISON OF ASSOCIATION INDICES Running Head: Cairns & Schwager: Association Indices

Abstract Indices used to meas ~ the frequency of association between individuals in fission/fusion societies are frequently borrowed from ecological studies of species association without adequate justification. This paper examines several such indices under specific conditions likely to be encountered in field studies of animal behaviour. Each of three indices commonly found in the literature is shown through simulations to be accurate over only a narrow range of possible sampling biases. As an alternative approach, examples are given of the derivation of a maximum likelihood estimator based on two simple models: one assuming that a constant proportion of existing subgroups is located, the other allowing for differential visibility of subgroups. The maximum likelihood estimators are shown to 9e less biased and to have lower variance than the · other three indices under the assumptions of the models.