Word Trigram Model (Fluency)

We can evaluate English sentences on two points; adequacy and fluency. We believe that fluency is related to language model $P(E)$. Thus we used a normal trigram model and did not use a higher $N$-gram model. In general, when we use a higher order $N$-gram, the number of parameters dramatically increase. It occurs that, the reliability for each parameter becomes low.

We believe that a trigram model is the best language model to express fluency.