Although many useful smoothing techniques are developed to estimate these unseen sequences, it is still important to make full use of contextual information in training data.
a number of useful smoothing techniques such as back-off (Katz,1987), Kneser-Ney (Kneser & Ney,1995), modified Kneser-Ney (Chen & Goodman,1999) have been developed to estimate the probabilities of unseen sequences.