# 一种机器翻译的评价准则——Bleu

### 2. 论文中使用的例子

#### Example 1.

Candidate 1：It is a guide to action which ensures that the military always obeys the commands of the party.

Candidate 2: It is to insure the troops forever hearing the activity guidebook that party direct.

Reference 1: It is a guide to action that ensures that the military will forever heed Party commands.

Reference 2: It is the guiding principle which guarantees the military forces always being under the command of the Party.

Reference 3: It is the practical guide for the army always to heed the directions of the party .

#### Example 2.

Candidate: the the the the the the the.

Reference 1: The cat is on the mat.

Reference 2: There is a cat on the mat.

#### Example 3.

Candidate: of the

Reference 1: It is a guide to action that ensures that the military will forever heed Party commands.

Reference 2: It is the guiding principle which guarantees the military forces always being under the command of the Party.

Reference 3: It is the practical guide for the army always to heed the directions of the party.

#### Example 4.

Candidate 1: I always invariably perpetually do.

Candidate 2: I always do.

Reference 1: I always do.

Reference 2: I invariably do.

Reference 3: I perpetually do.

### 3. Bleu方法使用的基本度量指标和概念

#### 3.2 精确度（Precision）和“修正的n-单位精确度”(modified n-gram recision)

Precision是指Candidate语句里面的n-gram在所有Reference语句里面出现的概率。

pn=∑C∈{Candidate}∑n-gram∈CCountclip(n-gram)∑C'∈{Candidate}∑n-gram'∈C'Count(n-gram)

p_n = \frac {\sum_{C \in \left\{Candidate \right\}} \sum_{n\text{-}gram \in C}Count_{clip}(n\text{-}gram)}{\sum_{C^{\text{'}} \in \left\{Candidate \right\}} \sum_{n\text{-}gram\text{'} \in C\text{'}}Count(n\text{-}gram)} 简而言之，就是把所有句子的modified n-gram precision的分子加起来除以分母加起来。

#### 4. BP值(Brevity Penalty)和BLEU值的计算公式

BP={1 if c>re1−r/c if c≤r

BP = \begin{cases} 1 ~~ if ~~ c>r \\ e^{1-r/c}~~if~~c\leq r \end{cases} 之后又Bleu值等于

Bleu=BP⋅exp(∑n=1Nwnlogpn)

Bleu = BP \cdot\exp(\sum_{n=1}^N w_n\log p_n) 在对数情况下，计算变得更加简便

logBlue=min(1−rc,0)+∑n=1Nwnlogpn

\log Blue = \min(1-\frac r c,0)+\sum_{n=1}^N w_n\log p_n 通常这个N取4，wn=1/4w_n=1/4，这就是很多论文里面的一个经典指标Bleu4

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