100 lines
3.5 KiB
Python
100 lines
3.5 KiB
Python
"""
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=============================================================
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Receiver Operating Characteristic (ROC) with cross validation
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=============================================================
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Example of Receiver Operating Characteristic (ROC) metric to evaluate
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classifier output quality using cross-validation.
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ROC curves typically feature true positive rate on the Y axis, and false
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positive rate on the X axis. This means that the top left corner of the plot is
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the "ideal" point - a false positive rate of zero, and a true positive rate of
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one. This is not very realistic, but it does mean that a larger area under the
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curve (AUC) is usually better.
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The "steepness" of ROC curves is also important, since it is ideal to maximize
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the true positive rate while minimizing the false positive rate.
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This example shows the ROC response of different datasets, created from K-fold
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cross-validation. Taking all of these curves, it is possible to calculate the
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mean area under curve, and see the variance of the curve when the
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training set is split into different subsets. This roughly shows how the
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classifier output is affected by changes in the training data, and how
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different the splits generated by K-fold cross-validation are from one another.
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.. note::
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See also :func:`sklearn.metrics.roc_auc_score`,
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:func:`sklearn.model_selection.cross_val_score`,
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:ref:`sphx_glr_auto_examples_model_selection_plot_roc.py`,
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"""
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print(__doc__)
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import numpy as np
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import matplotlib.pyplot as plt
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from sklearn import svm, datasets
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from sklearn.metrics import auc
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from sklearn.metrics import plot_roc_curve
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from sklearn.model_selection import StratifiedKFold
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# #############################################################################
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# Data IO and generation
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# Import some data to play with
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iris = datasets.load_iris()
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X = iris.data
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y = iris.target
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X, y = X[y != 2], y[y != 2]
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n_samples, n_features = X.shape
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# Add noisy features
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random_state = np.random.RandomState(0)
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X = np.c_[X, random_state.randn(n_samples, 200 * n_features)]
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# #############################################################################
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# Classification and ROC analysis
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# Run classifier with cross-validation and plot ROC curves
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cv = StratifiedKFold(n_splits=6)
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classifier = svm.SVC(kernel='linear', probability=True,
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random_state=random_state)
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tprs = []
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aucs = []
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mean_fpr = np.linspace(0, 1, 100)
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fig, ax = plt.subplots()
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for i, (train, test) in enumerate(cv.split(X, y)):
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classifier.fit(X[train], y[train])
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viz = plot_roc_curve(classifier, X[test], y[test],
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name='ROC fold {}'.format(i),
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alpha=0.3, lw=1, ax=ax)
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interp_tpr = np.interp(mean_fpr, viz.fpr, viz.tpr)
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interp_tpr[0] = 0.0
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tprs.append(interp_tpr)
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aucs.append(viz.roc_auc)
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ax.plot([0, 1], [0, 1], linestyle='--', lw=2, color='r',
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label='Chance', alpha=.8)
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mean_tpr = np.mean(tprs, axis=0)
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mean_tpr[-1] = 1.0
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mean_auc = auc(mean_fpr, mean_tpr)
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std_auc = np.std(aucs)
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ax.plot(mean_fpr, mean_tpr, color='b',
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label=r'Mean ROC (AUC = %0.2f $\pm$ %0.2f)' % (mean_auc, std_auc),
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lw=2, alpha=.8)
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std_tpr = np.std(tprs, axis=0)
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tprs_upper = np.minimum(mean_tpr + std_tpr, 1)
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tprs_lower = np.maximum(mean_tpr - std_tpr, 0)
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ax.fill_between(mean_fpr, tprs_lower, tprs_upper, color='grey', alpha=.2,
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label=r'$\pm$ 1 std. dev.')
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ax.set(xlim=[-0.05, 1.05], ylim=[-0.05, 1.05],
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title="Receiver operating characteristic example")
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ax.legend(loc="lower right")
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plt.show()
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