Multi-Source Neural Translation
Figure 4: Multi-source MT for target English, with source languages French and German. Ppl reports test-set perplexity as the system predicts English tokens. BLEU is scored using the multi-bleu.perl script from Moses. For our evaluation we use a single reference and they are case sensitive.
['Target = English  [BOLD] Source', 'Target = English  [BOLD] Method', 'Target = English  [BOLD] Ppl', 'Target = English  [BOLD] BLEU']
We see that the Basic combination method yields a +4.8 Bleu improvement over the strongest single-source, attention-based system. It also improves Bleu by +2.2 over the non-attention baseline. The Child-Sum method gives improvements of +4.4 and +1.4. We also confirm that two copies of the same French input yields no BLEU improvement.