Question

我正在寻找一个关于如何运行Multinomial朴素贝叶斯分类器的简单示例。我从StackOverflow中看到了这个例子：

Implementing Bag-of-Words Naive-Bayes classifier in NLTK

import numpy as np
from nltk.probability import FreqDist
from nltk.classify import SklearnClassifier
from sklearn.feature_extraction.text import TfidfTransformer
from sklearn.feature_selection import SelectKBest, chi2
from sklearn.naive_bayes import MultinomialNB
from sklearn.pipeline import Pipeline

pipeline = Pipeline([('tfidf', TfidfTransformer()),
                     ('chi2', SelectKBest(chi2, k=1000)),
                     ('nb', MultinomialNB())])
classif = SklearnClassifier(pipeline)

from nltk.corpus import movie_reviews
pos = [FreqDist(movie_reviews.words(i)) for i in movie_reviews.fileids('pos')]
neg = [FreqDist(movie_reviews.words(i)) for i in movie_reviews.fileids('neg')]
add_label = lambda lst, lab: [(x, lab) for x in lst]
#Original code from thread:
#classif.train(add_label(pos[:100], 'pos') + add_label(neg[:100], 'neg'))
classif.train(add_label(pos, 'pos') + add_label(neg, 'neg'))#Made changes here

#Original code from thread:    
#l_pos = np.array(classif.batch_classify(pos[100:]))
#l_neg = np.array(classif.batch_classify(neg[100:]))
l_pos = np.array(classif.batch_classify(pos))#Made changes here
l_neg = np.array(classif.batch_classify(neg))#Made changes here
print "Confusion matrix:\n%d\t%d\n%d\t%d" % (
          (l_pos == 'pos').sum(), (l_pos == 'neg').sum(),
          (l_neg == 'pos').sum(), (l_neg == 'neg').sum())

运行此示例后，我收到了警告。

C:\Python27\lib\site-packages\scikit_learn-0.13.1-py2.7-win32.egg\sklearn\feature_selection\univariate_selection.py:327: 
UserWarning: Duplicate scores. Result may depend on feature ordering.There are probably duplicate features, 
or you used a classification score for a regression task.
warn("Duplicate scores. Result may depend on feature ordering."

Confusion matrix:
876 124
63  937

所以，我的问题是......

有谁能告诉我这个错误信息是什么意思？
我对原始代码进行了一些更改，但为什么混淆矩阵的结果比原始代码中的结果要高得多呢？
如何测试此分类器的准确性？

Answer 1

原始代码训练前100个正面和负面的例子，然后对剩余部分进行分类。您已经删除了边界并在训练和分类阶段使用了每个示例，换句话说，您有重复的功能。要解决此问题，请将数据集拆分为两组，训练和测试。

混淆矩阵更高（或不同），因为您正在训练不同的数据。

混淆矩阵是衡量准确度的指标，显示误报的数量等。阅读更多内容：http://en.wikipedia.org/wiki/Confusion_matrix

Answer 2

我使用的原始代码仅包含训练集的前100个条目，但仍然有警告。我的输出是：

In [6]: %run testclassifier.py
C:\Users\..\AppData\Local\Enthought\Canopy\User\lib\site-packages\sklearn\feature_selection\univariate_selecti
on.py:319: UserWarning: Duplicate scores. Result may depend on feature ordering.There are probably duplicate features, o
r you used a classification score for a regression task.
  warn("Duplicate scores. Result may depend on feature ordering."
Confusion matrix:
427     473
132     768

用Python实例分类多项式朴素贝叶斯分类器

2 个答案: