The second way is randomly sampling a small set of s ′ with respect to its groundtruth ROUGE score (Line 6 of Algorithm 1).
for each training episode of sentence s, we sample a small set S ′ 2 according to the following rule: where rnd(0, 1) generates random number from a uniform distribution within the range Each sentence is selected with at least 5% probability and sentences with higher ROUGE scores have higher probabilities.