BLEU Score

BLEU (Bilingual Evaluation Understudy) is an automatic evaluation metric originally designed for machine translation that measures how similar a machine-generated text is to a human-written reference. Scores range from 0 to 1, with higher scores indicating closer matches.

How BLEU works

BLEU computes the overlap of n-grams — sequences of consecutive words — between the generated text and reference texts. It checks how many unigrams (single words), bigrams (two-word sequences), trigrams, and four-grams in the generated text also appear in the reference.

For example, if the reference is "The cat sat on the mat" and the machine produces "The cat is on the mat," BLEU would note that most words match but "is" does not appear in the reference, reducing the score.

BLEU also includes a brevity penalty — if the generated text is much shorter than the reference, the score is reduced to prevent a system from gaming the metric by only outputting words it is confident about.

Strengths of BLEU

• Fast and cheap: BLEU is computed automatically with no human judges needed.
• Reproducible: The same inputs always produce the same score.
• Widely understood: Decades of research use BLEU, making it a common baseline.

Limitations of BLEU

• Surface-level matching: BLEU only checks word overlap. "The dog is big" and "The canine is large" have low BLEU overlap despite identical meaning.
• No understanding of meaning: A grammatically broken sentence with the right words can score higher than a perfect paraphrase.
• Poor for open-ended generation: BLEU assumes there is a "correct" reference. For creative writing, summarization, or conversation, there are many valid outputs that would score poorly.

Modern alternatives

Researchers have developed more sophisticated metrics like ROUGE (for summarization), BERTScore (using semantic similarity), and human preference ratings. Many teams now use LLM-as-judge approaches, where a capable language model evaluates output quality. Despite its limitations, BLEU remains a useful quick benchmark for translation and structured generation tasks.