Traditional approaches to zero anaphora resolution are based on manually created heuristic rules (Kameyama, 1986;Walker et al., 1994;Okumura and Tamura, 1996;Nakaiwa and Shirai, 1996), which are mainly motivated by the rules and preferences introduced in Centering Theory (Grosz et al., 1995).
the research trend of zero anaphora resolution has shifted from such rule-based approaches to machine learningbased approaches because in machine learning we can easily integrate many different types of information, such as morpho-syntactic, semantic and discourse-related information.