Task: Question Answering
Input (English): Intuitively, under-translated input words should contribute little to the NMT outputs, yielding much smaller word importance.  By exploiting the word importance calculated by Attribution method, we can identify the under-translation errors automatically without the involvement of human interpreters.  
 Question: How do they measure which words are under-translated by NMT models?
Output (English): They measured the under-translated words with low word importance score as calculated by Attribution.
method