The results obtained by Method 2 and Method 3 are similar, since both of them consider the QI when given the set of selected sentence pairs S only.
when comparing the Method 2 and 3 with Method 4, we find Method 2 and 3 obtained better results when selecting only 10% and 30% of the corpus, and after increasing the ratios, method 4 obtains the better results, especially it obtains the best results when selecting only 80% of the corpus.