python - scikit 学习决策树模型评估

Question

下面是相关的代码和文档，想知道默认的cross_val_score没有明确指定score，输出数组是指精度，AUC还是其他一些指标？

将 Python 2.7 与 miniconda 解释器结合使用。

http://scikit-learn.org/stable/modules/generated/sklearn.tree.DecisionTreeClassifier.html

>>> from sklearn.datasets import load_iris
>>> from sklearn.cross_validation import cross_val_score
>>> from sklearn.tree import DecisionTreeClassifier
>>> clf = DecisionTreeClassifier(random_state=0)
>>> iris = load_iris()
>>> cross_val_score(clf, iris.data, iris.target, cv=10)
...                             
...
array([ 1.     ,  0.93...,  0.86...,  0.93...,  0.93...,
        0.93...,  0.93...,  1.     ,  0.93...,  1.      ])

问候，林

Answer 1

来自user guide :

By default, the score computed at each CV iteration is the score method of the estimator. It is possible to change this by using the scoring parameter:

来自决策树分类器 documentation :

Returns the mean accuracy on the given test data and labels. In multi-label classification, this is the subset accuracy which is a harsh metric since you require for each sample that each label set be correctly predicted.

不要对“平均准确度”感到困惑，这只是计算准确度的常规方法。点击链接到 source :

    from .metrics import accuracy_score
    return accuracy_score(y, self.predict(X), sample_weight=sample_weight)

现在 source对于 metrics.accuracy_score

def accuracy_score(y_true, y_pred, normalize=True, sample_weight=None):
    ...
    # Compute accuracy for each possible representation
    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
    if y_type.startswith('multilabel'):
        differing_labels = count_nonzero(y_true - y_pred, axis=1)
        score = differing_labels == 0
    else:
        score = y_true == y_pred

    return _weighted_sum(score, sample_weight, normalize)

如果你 still aren't convinced:

def _weighted_sum(sample_score, sample_weight, normalize=False):
    if normalize:
        return np.average(sample_score, weights=sample_weight)
    elif sample_weight is not None:
        return np.dot(sample_score, sample_weight)
    else:
        return sample_score.sum()

注意:对于 accuracy_score 规范化参数默认为 True，因此它只是返回 bool numpy 数组的 np.average，因此它很简单正确预测的平均数。