Because it is well known that handling unknown words is a serious problem for POS taggers, our strategy for selecting sentences for manual annotation is to maximize the word coverage of the in ital model.
we wish to find a small set of sentences that would lead to the greatest reduction of currently unknown words Finding these sentences is a NP-hard problem because the 0/1 knapsack problem could be reduced to this problem in polynomial-time (Gurari, 1989).