 Brevity penalty

The brevity penalty in BLEU reduces the score of a candidate translation corpus that is too short relative to its references. Matching a few reference phrases can give excellent modified n-gram precision while omitting much of the intended translation; the length factor addresses that particular failure.^[ref]

For positive total candidate length $c$ and effective reference length $r$,Equivalently, $\log \mathrm{BP}=\min \{0,1-r/c\}$. The effective reference length sums closest reference sentence lengths across the corpus; the penalty is then applied to the corpus totals.^[ref]

For an own example, use reference the cat is on the mat and candidate the cat is on. Every candidate n-gram of lengths one through four matches the reference, so all four modified precisions are one. Nevertheless, $c=4$ and $r=6$ giveWith the usual four equal weights, the full score in this example is also $0.6065$, because its precision geometric mean is one. Perfect candidate precision has not made an incomplete prefix a perfect translation.

At $c=r$, the factor is one. It remains one for longer candidates; their unmatched or repeated material can instead lower precision. A brevity penalty is therefore a length correction within a combined metric, not a complete measure of translation adequacy.

Report how reference lengths are selected and how empty outputs are handled before comparing implementations. The displayed division assumes $c>0$. Also distinguish corpus scoring from averaging sentence penalties, which need not produce the same result.