An approach based on pairwise similarities, which encourage nearby data points to have the same class label, has been proposed as a way of incorporating unlabeled data discriminatively (Zhu et al., 2003;Altun et al., 2005;Brefeld and Scheffer, 2006).
this approach generally requires joint inference over the whole data set for prediction, which is not practical as regards the large data sets used for standard sequence labeling tasks in NLP.