Majority of the research focussed on machine learning (ML) approaches (Bikel et al., 1999;Borthwick, 1999;Sekine, 1998;Lafferty et al., 2001a;Yamada et al., 2001) because these are easily trainable, adaptable to different domains and languages as well as their maintenance are also being less expensive.
rule-based approaches lack the ability of dealing with the problems of robustness and portability.