Is a higher BLEU score always better?
No, a higher BLEU score does not always equate to better translation quality. While it's a widely used automated metric, it has significant limitations.
BLEU measures n-gram overlap between a machine translation output and one or more human reference translations. It correlates reasonably well with human judgment in aggregate comparisons. However, it fails to capture semantic adequacy, fluency, or stylistic appropriateness. Over-optimizing solely for BLEU can lead to outputs that match references superficially but lack coherence or meaning. The metric is also highly sensitive to reference quality, domain specificity, and text length.
Primarily useful for system-level comparison during development, BLEU offers efficiency. It is not reliable for evaluating individual sentences or for nuanced quality distinctions. Achieving a high score does not guarantee a genuinely good translation. Human evaluation remains the essential complement for assessing true translation quality and usefulness, as BLEU cannot fully comprehend meaning or context.
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