BLEU Score › Interpreting BLEU Scores in Practice
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Understanding what BLEU scores mean in practice requires context. The table below provides rough guidelines for interpreting corpus-level BLEU scores in standard news domain machine translation benchmarks, based on the field's accumulated experience:
These ranges are approximate and highly domain-dependent. A BLEU of 35 is exceptional for Japanese-English literary translation but mediocre for Spanish-English news. Always compare against the best published baseline for your specific language pair and domain.
When evaluating your own system, the most useful interpretation of BLEU is relative, not absolute. A 2-point improvement over a competitive baseline on a well-studied benchmark is meaningful, while a 10-point improvement on a dataset of your own creation may reflect dataset construction choices more than real progress. The research community has learned, sometimes painfully, that BLEU improvements on held-out test sets do not always generalize to user satisfaction in production systems.