Owing to the presence of hidden data, we estimate the model parameters using the Expectation-Maximization (EM) algorithm (Dempster et al., 1977).
we use EM to iteratively estimate the model parameters from data in which each event mention is labeled with the probability that it corefers with each of its candidate antecedents, and apply the resulting model to relabel each event mention with the probability that it corefers with each of its candidate antecedents.