Example of confusion matrix usage to evaluate the quality of the output of a classifier on the iris data set. The diagonal elements represent the number of points for which the predicted label is equal to the true label, while off-diagonal elements are those that are mislabeled by the classifier. The higher the diagonal values of the confusion matrix the better, indicating many correct predictions.
The figures show the confusion matrix with and without normalization by class support size (number of elements in each class). This kind of normalization can be interesting in case of class imbalance to have a more visual interpretation of which class is being misclassified.
Here the results are not as good as they could be as our choice for the regularization parameter C was not the best. In real life applications this parameter is usually chosen using Tuning the hyper-parameters of an estimator.
Confusion matrix, without normalization [[13 0 0] [ 0 10 6] [ 0 0 9]] Normalized confusion matrix [[1. 0. 0. ] [0. 0.62 0.38] [0. 0. 1. ]]
import matplotlib.pyplot as plt import numpy as np from sklearn import datasets, svm from sklearn.metrics import ConfusionMatrixDisplay from sklearn.model_selection import train_test_split # import some data to play with iris = datasets.load_iris() X = iris.data y = iris.target class_names = iris.target_names # Split the data into a training set and a test set X_train, X_test, y_train, y_test = train_test_split(X, y, random_state=0) # Run classifier, using a model that is too regularized (C too low) to see # the impact on the results classifier = svm.SVC(kernel="linear", C=0.01).fit(X_train, y_train) np.set_printoptions(precision=2) # Plot non-normalized confusion matrix titles_options = [ ("Confusion matrix, without normalization", None), ("Normalized confusion matrix", "true"), ] for title, normalize in titles_options: disp = ConfusionMatrixDisplay.from_estimator( classifier, X_test, y_test, display_labels=class_names, cmap=plt.cm.Blues, normalize=normalize, ) disp.ax_.set_title(title) print(title) print(disp.confusion_matrix) plt.show()
Total running time of the script: (0 minutes 0.223 seconds)