However, a modified estimate is used for lower order n-grams used for back-off.
the interpolated form is: Pr(c|ab) = α(c|ab) + γ(ab)Pr (c|b) (5) Pr (c|ab) = α (c|ab) + γ (ab)Pr (c|b) the modified estimate Pr (c|b) for a lower order n-gram is taken to be proportional to the number of unique words that precede the n-gram in the training data.