We experiment with choices of n from 2 to 4, and two popular smoothing approaches, Modified Kneser-Ney (Chen and Goodman, 1996) and Witten-Bell (Bell et al., 1990) . Unsupervised Hidden Markov Models (HMMs) are an alternative SLM approach previously shown to offer accuracy and scalability advantages over ngram models in ADS <cite>(Downey et al., 2007)</cite> .