3 research outputs found
HUME: Human UCCA-Based Evaluation of Machine Translation
Human evaluation of machine translation normally uses sentence-level measures
such as relative ranking or adequacy scales. However, these provide no insight
into possible errors, and do not scale well with sentence length. We argue for
a semantics-based evaluation, which captures what meaning components are
retained in the MT output, thus providing a more fine-grained analysis of
translation quality, and enabling the construction and tuning of
semantics-based MT. We present a novel human semantic evaluation measure, Human
UCCA-based MT Evaluation (HUME), building on the UCCA semantic representation
scheme. HUME covers a wider range of semantic phenomena than previous methods
and does not rely on semantic annotation of the potentially garbled MT output.
We experiment with four language pairs, demonstrating HUME's broad
applicability, and report good inter-annotator agreement rates and correlation
with human adequacy scores
Syntactic and semantic features for statistical and neural machine translation
Machine Translation (MT) for language pairs with long distance dependencies and
word reordering, such as German–English, is prone to producing output that is lexically
or syntactically incoherent. Statistical MT (SMT) models used explicit or latent
syntax to improve reordering, however failed at capturing other long distance dependencies.
This thesis explores how explicit sentence-level syntactic information can improve
translation for such complex linguistic phenomena. In particular, we work at the
level of the syntactic-semantic interface with representations conveying the predicate-argument
structures. These are essential to preserving semantics in translation and
SMT systems have long struggled to model them.
String-to-tree SMT systems use explicit target syntax to handle long-distance reordering,
but make strong independence assumptions which lead to inconsistent lexical
choices. To address this, we propose a Selectional Preferences feature which models
the semantic affinities between target predicates and their argument fillers using the
target dependency relations available in the decoder. We found that our feature is not
effective in a string-to-tree system for German→English and that often the conditioning
context is wrong because of mistranslated verbs.
To improve verb translation, we proposed a Neural Verb Lexicon Model (NVLM)
incorporating sentence-level syntactic context from the source which carries relevant
semantic information for verb disambiguation. When used as an extra feature for re-ranking
the output of a German→ English string-to-tree system, the NVLM improved
verb translation precision by up to 2.7% and recall by up to 7.4%.
While the NVLM improved some aspects of translation, other syntactic and lexical
inconsistencies are not being addressed by a linear combination of independent models.
In contrast to SMT, neural machine translation (NMT) avoids strong independence
assumptions thus generating more fluent translations and capturing some long-distance
dependencies. Still, incorporating additional linguistic information can improve translation
quality.
We proposed a method for tightly coupling target words and syntax in the NMT
decoder. To represent syntax explicitly, we used CCG supertags, which encode subcategorization
information, capturing long distance dependencies and attachments. Our
method improved translation quality on several difficult linguistic constructs, including
prepositional phrases which are the most frequent type of predicate arguments. These
improvements over a strong baseline NMT system were consistent across two language
pairs: 0.9 BLEU for German→English and 1.2 BLEU for Romanian→English