Modeling Source Syntax for Neural Machine Translation
Table 4: Ratio of over translation (ROT) on test sets. Here NR is for proper noun, CD for cardinal number, DT for determiner, and NN for nouns except proper nouns and temporal nouns.
['[BOLD] System', '[BOLD] POS', '[BOLD] ROT (%)']
It is not surprising that RNNSearch system has high ROT with respect to POS tags of NR (proper noun) and CD (cardinal number): this is due to the fact that the two POS tags include high percentage of unknown words which tend to be translated multiple times in translation. Words of DT (determiner) are another source of over translation since they are usually translated to multiple the in English. It also shows that by introducing source syntax, Mixed RNN system alleviates the over translation issue by 18%: ROT drops from 5.5% to 4.5%.