Discriminative Models for Semi-Supervised Natural Language Learning

An interesting question surrounding semisupervised learning for NLP is: should we use discriminative models or generative models? Despite the fact that generative models have been frequently employed in a semi-supervised setting since the early days of the statistical revolution in NLP, we advocate the use of discriminative models. The ability of discriminative models to handle complex, high-dimensional feature spaces and their strong theoretical guarantees have made them a very appealing alternative to their generative counterparts. Perhaps more importantly, discriminative models have been shown to offer competitive performance on a variety of sequential and structured learning tasks in NLP that are traditionally tackled via generative models , such as letter-to-phoneme conversion (Jiampojamarn et al., 2008), semantic role labeling (Toutanova et al., 2005), syntactic parsing (Taskar et al., 2004), language modeling (Roark et al., 2004), and machine translation (Liang et al., 2006). While generative models allow the seamless integration of prior knowledge, discriminative models seem to outperform generative models in a “no prior”, agnostic learning setting. See Ng and Jordan (2002) and Toutanova (2006) for insightful comparisons of generative and discriminative models.