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arXiv 2005.10283cs.CL
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MAP是否能解码您所需要的一切?神经机器翻译中模式的不足

Is MAP Decoding All You Need? The Inadequacy of the Mode in Neural Machine Translation

University of Amsterdam(阿姆斯特丹大学)

Bryan Eikema, Wilker Aziz

摘要:

最近的研究揭示了神经机器翻译(NMT)系统的一些病理。解释这些的假设大多表明NMT作为模型或其训练算法,最大似然估计(MLE)存在根本性的错误。大多数证据都是使用最大后验(MAP)解码收集的,这是一种旨在识别得分最高的翻译(即模式)的决策规则。我们认为,证据证实了MAP解码的不足,而不是对模型及其训练算法提出质疑。在这项工作中,我们表明,平移分布再现各种统计数据以及,但束搜索偏离这样的统计。我们发现,一些已知的病理和偏见的NMT是由于MAP解码,而不是NMT的统计假设,也没有MLE。特别是,我们表明,最有可能的模型下的翻译积累这么少的概率质量,该模式可以被认为是基本上任意的。因此,我们提倡使用从整体上考虑翻译分布的决策规则。我们表明,对最小贝叶斯风险解码的逼近给出了具有竞争力的结果,证实NMT模型确实在预期中很好地捕捉到了翻译的重要方面。

英文摘要:

Recent studies have revealed a number of pathologies of neural machine translation (NMT) systems. Hypotheses explaining these mostly suggest there is something fundamentally wrong with NMT as a model or its training algorithm, maximum likelihood estimation (MLE). Most of this evidence was gathered using maximum a posteriori (MAP) decoding, a decision rule aimed at identifying the highest-scoring translation, i.e. the mode. We argue that the evidence corroborates the inadequacy of MAP decoding more than casts doubt on the model and its training algorithm. In this work, we show that translation distributions do reproduce various statistics of the data well, but that beam search strays from such statistics. We show that some of the known pathologies and biases of NMT are due to MAP decoding and not to NMT's statistical assumptions nor MLE. In particular, we show that the most likely translations under the model accumulate so little probability mass that the mode can be considered essentially arbitrary. We therefore advocate for the use of decision rules that take into account the translation distribution holistically. We show that an approximation to minimum Bayes risk decoding gives competitive results confirming that NMT models do capture important aspects of translation well in expectation.

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