I'm reading the paper on the Bilingual Evaluation Understudy (BLEU) metric BLEU: A Method for Automatic Evaluation of Machine Translation (Papineni et al., 2002) and had a question regarding a quote in the paper.
The quote is motivated by the observation that good translation metrics should also take the length of the translation into account. Until now, the main focus of the paper had been modified n-gram precision, which is unable to properly account for translation candidates that are either too short or too long.
The paper states that:
Traditionally, precision has been paired with recall to overcome such length-related problems. However, BLEU considers multiple reference translations, each of which may use a different word choice to translate the same source word.
I'm having some trouble understanding that statement. My understanding of recall is that it adds false negatives rather than false positives in the denominator. In a more typical situation, I can imagine that a high precision due to low false positives could be counteracted with a low recall due to many false negatives. However, I'm having trouble how we can apply that concept to translation. I'm also having trouble understanding how using multiple reference translations would hinder us from using recall to counteract n-gram precision.
Any tips are appreciated. Thanks.