The Edit Distance Transducer in Action: The University of Cambridge English-German System at WMT16
Table 4: BLEU scores on news-test2016 for different vocabulary sizes (single NMT). Each individual NMT system is combined with Hiero as described in Sec. 2.3.
['[BOLD] Vocabulary  [BOLD] size', '[BOLD] Pure NMT  [BOLD] BLEU', '[BOLD] Pure NMT  [BOLD] # of UNKs', '[BOLD] NMT+Hiero  [BOLD] BLEU']
To study the effectiveness of our edit distance transducer based combination scheme in correcting NMT UNKs, we trained individual NMT systems with vocabulary sizes between 10,000 and 60,000. Tab. Increasing the NMT vocabulary to 50k or 60k does improve pure NMT very significantly, but results show that these improvements are already captured by the combination scheme with Hiero.