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Python Machine Learning By Example
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So far, we have covered the first machine learning classifier and evaluated its performance by prediction accuracy in-depth. Beyond accuracy, there are several measurements that give us more insights and avoid class imbalance effects.
Confusion matrix summarizes testing instances by their predicted values and true values, presented as a contingency table:
To illustrate, we compute the confusion matrix of our naive Bayes classifier. Here the scikit-learn confusion_matrix
function is used, but it is very easy to code it ourselves:
>>> from sklearn.metrics import confusion_matrix >>> confusion_matrix(Y_test, prediction, labels=[0, 1]) array([[1098, 93], [ 43, 473]])
Note that we consider 1
the spam class to be positive. From the confusion matrix, for example, there are 93 false positive cases (where it misinterprets a legitimate email as a spam one), and 43 false negative cases (where it fails to detect a spam email). And the classification...