Robust Learning, Smoothing, and Parameter Tying on Syntactic Ambiguity Resolution

Statistical approaches to natural language processing generally obtain the parameters by using the maximum likelihood estimation (MLE ) method. The MLE approaches, however, may fail to achieve good performance in difficult tasks, because the discrimination and robustness issues are not taken into consideration in the estimation processes. Motivated by that concern, a discriminationand robustness-oriented learning algorithm is proposed in this paper for minimizing the error rate. In evaluating the robust learning procedure on a corpus of 1,000 sentences, 64.3% of the sentences are assigned their correct syntactic structures, while only 53.1% accuracy rate is obtained with the MLE approach. In addition, parameters are usually estimated poorly when the training data is sparse. Smoothing the parameters is thus important in the estimation process. Accordingly, we use a hybrid approach combining the robust learning procedure with the smoothing method. The accuracy rate of 69.8% is attained by using this approach. Finally, a parameter tying scheme is proposed to tie those highly correlated but unreliably estimated parameters together so that the parameters can be better trained in the learning process. With this tying scheme, the number of parameters is reduced by a factor of 2,000 (from 8.7 x 108 to 4.2 x lOS), and the accuracy rate for parse tree selection is improved up to 70.3% when the robust learning procedure is applied on the tied parameters.