Different approaches vary in how this probabilistic network is generated and which method is applied to combine individual distributions. Maximum Likelihood Estimation and Bayesian approaches are typical examples. Rule-based approaches require matching to pre-defined r es or heuristic patterns in order to extract terms and relations. A rule-based model is typically represented as a set of rules consisting of condition testing and action execution, such as dependency relation analysis and anaphoric resolution. Hybrid approaches leverage the strengths of both statistics-based and rulebased approaches.