The existence of maximum likelihood estimates in the Bradley-Terry model and its extensions

Abstract

In the Bradley-Terry model for paired comparisons, and its extensions to include order effects and ties, the maximum likelihood estimates of probabilities of certain outcomes can be 0 or 1 under certain data configurations. This poses problems for standard estimation methods. In this paper, we give algorithms for identifying the outcomes with estimated probability 0 or 1, and indicate how the remaining probabilities may be estimated and summarized.