.. _sphx_glr_auto_examples_calibration_plot_calibration.py: ====================================== Probability calibration of classifiers ====================================== When performing classification you often want to predict not only the class label, but also the associated probability. This probability gives you some kind of confidence on the prediction. However, not all classifiers provide well-calibrated probabilities, some being over-confident while others being under-confident. Thus, a separate calibration of predicted probabilities is often desirable as a postprocessing. This example illustrates two different methods for this calibration and evaluates the quality of the returned probabilities using Brier's score (see https://en.wikipedia.org/wiki/Brier_score). Compared are the estimated probability using a Gaussian naive Bayes classifier without calibration, with a sigmoid calibration, and with a non-parametric isotonic calibration. One can observe that only the non-parametric model is able to provide a probability calibration that returns probabilities close to the expected 0.5 for most of the samples belonging to the middle cluster with heterogeneous labels. This results in a significantly improved Brier score. .. code-block:: python print(__doc__) # Author: Mathieu Blondel # Alexandre Gramfort # Balazs Kegl # Jan Hendrik Metzen # License: BSD Style. import numpy as np import matplotlib.pyplot as plt from matplotlib import cm from sklearn.datasets import make_blobs from sklearn.naive_bayes import GaussianNB from sklearn.metrics import brier_score_loss from sklearn.calibration import CalibratedClassifierCV from sklearn.model_selection import train_test_split n_samples = 50000 n_bins = 3 # use 3 bins for calibration_curve as we have 3 clusters here # Generate 3 blobs with 2 classes where the second blob contains # half positive samples and half negative samples. Probability in this # blob is therefore 0.5. centers = [(-5, -5), (0, 0), (5, 5)] X, y = make_blobs(n_samples=n_samples, n_features=2, cluster_std=1.0, centers=centers, shuffle=False, random_state=42) y[:n_samples // 2] = 0 y[n_samples // 2:] = 1 sample_weight = np.random.RandomState(42).rand(y.shape[0]) # split train, test for calibration X_train, X_test, y_train, y_test, sw_train, sw_test = \ train_test_split(X, y, sample_weight, test_size=0.9, random_state=42) # Gaussian Naive-Bayes with no calibration clf = GaussianNB() clf.fit(X_train, y_train) # GaussianNB itself does not support sample-weights prob_pos_clf = clf.predict_proba(X_test)[:, 1] # Gaussian Naive-Bayes with isotonic calibration clf_isotonic = CalibratedClassifierCV(clf, cv=2, method='isotonic') clf_isotonic.fit(X_train, y_train, sw_train) prob_pos_isotonic = clf_isotonic.predict_proba(X_test)[:, 1] # Gaussian Naive-Bayes with sigmoid calibration clf_sigmoid = CalibratedClassifierCV(clf, cv=2, method='sigmoid') clf_sigmoid.fit(X_train, y_train, sw_train) prob_pos_sigmoid = clf_sigmoid.predict_proba(X_test)[:, 1] print("Brier scores: (the smaller the better)") clf_score = brier_score_loss(y_test, prob_pos_clf, sw_test) print("No calibration: %1.3f" % clf_score) clf_isotonic_score = brier_score_loss(y_test, prob_pos_isotonic, sw_test) print("With isotonic calibration: %1.3f" % clf_isotonic_score) clf_sigmoid_score = brier_score_loss(y_test, prob_pos_sigmoid, sw_test) print("With sigmoid calibration: %1.3f" % clf_sigmoid_score) .. rst-class:: sphx-glr-script-out Out:: Brier scores: (the smaller the better) No calibration: 0.104 With isotonic calibration: 0.084 With sigmoid calibration: 0.109 Plot the data and the predicted probabilities .. code-block:: python plt.figure() y_unique = np.unique(y) colors = cm.rainbow(np.linspace(0.0, 1.0, y_unique.size)) for this_y, color in zip(y_unique, colors): this_X = X_train[y_train == this_y] this_sw = sw_train[y_train == this_y] plt.scatter(this_X[:, 0], this_X[:, 1], s=this_sw * 50, c=color, alpha=0.5, label="Class %s" % this_y) plt.legend(loc="best") plt.title("Data") plt.figure() order = np.lexsort((prob_pos_clf, )) plt.plot(prob_pos_clf[order], 'r', label='No calibration (%1.3f)' % clf_score) plt.plot(prob_pos_isotonic[order], 'g', linewidth=3, label='Isotonic calibration (%1.3f)' % clf_isotonic_score) plt.plot(prob_pos_sigmoid[order], 'b', linewidth=3, label='Sigmoid calibration (%1.3f)' % clf_sigmoid_score) plt.plot(np.linspace(0, y_test.size, 51)[1::2], y_test[order].reshape(25, -1).mean(1), 'k', linewidth=3, label=r'Empirical') plt.ylim([-0.05, 1.05]) plt.xlabel("Instances sorted according to predicted probability " "(uncalibrated GNB)") plt.ylabel("P(y=1)") plt.legend(loc="upper left") plt.title("Gaussian naive Bayes probabilities") plt.show() .. rst-class:: sphx-glr-horizontal * .. image:: /auto_examples/calibration/images/sphx_glr_plot_calibration_001.png :scale: 47 * .. image:: /auto_examples/calibration/images/sphx_glr_plot_calibration_002.png :scale: 47 **Total running time of the script:** (0 minutes 0.272 seconds) .. container:: sphx-glr-download **Download Python source code:** :download:`plot_calibration.py ` .. container:: sphx-glr-download **Download IPython notebook:** :download:`plot_calibration.ipynb `