  The quality of Neural Machine Translation (NMT) has been shown to
significantly degrade when confronted with source-side noise. We present the
first large-scale study of state-of-the-art English-to-German NMT on real
grammatical noise, by evaluating on several Grammar Correction corpora. We
present methods for evaluating NMT robustness without true references, and we
use them for extensive analysis of the effects that different grammatical
errors have on the NMT output. We also introduce a technique for visualizing
the divergence distribution caused by a source-side error, which allows for
additional insights.
