sklearn scoring

3.3.1.1. Common cases: predefined values

For the most common use cases, you can designate a scorer object with the scoring parameter; the table below shows all possible values. All scorer objects follow the convention that higher return values are better than lower return values. Thus metrics which measure the distance between the model and the data, like metrics.mean_squared_error, are available as neg_mean_squared_error which return the negated value of the metric.

Usage examples:

>>> from sklearn import svm, datasets
>>> from sklearn.model_selection import cross_val_score
>>> iris = datasets.load_iris()
>>> X, y = iris.data, iris.target
>>> clf = svm.SVC(probability=True, random_state=0)
>>> cross_val_score(clf, X, y, scoring=‘neg_log_loss‘) 
array([-0.07..., -0.16..., -0.06...])
>>> model = svm.SVC()
>>> cross_val_score(model, X, y, scoring=‘wrong_choice‘)
Traceback (most recent call last):
ValueError: ‘wrong_choice‘ is not a valid scoring value. Valid options are [‘accuracy‘, ‘adjusted_rand_score‘, ‘average_precision‘, ‘f1‘, ‘f1_macro‘, ‘f1_micro‘, ‘f1_samples‘, ‘f1_weighted‘, ‘neg_log_loss‘, ‘neg_mean_absolute_error‘, ‘neg_mean_squared_error‘, ‘neg_median_absolute_error‘, ‘precision‘, ‘precision_macro‘, ‘precision_micro‘, ‘precision_samples‘, ‘precision_weighted‘, ‘r2‘, ‘recall‘, ‘recall_macro‘, ‘recall_micro‘, ‘recall_samples‘, ‘recall_weighted‘, ‘roc_auc‘]

3.3.1.2. Defining your scoring strategy from metric functions

The module sklearn.metric also exposes a set of simple functions measuring a prediction error given ground truth and prediction:

• functions ending with _score return a value to maximize, the higher the better.
• functions ending with _error or _loss return a value to minimize, the lower the better. When converting into a scorer object using make_scorer, set the greater_is_better parameter to False (True by default; see the parameter description below).

Metrics available for various machine learning tasks are detailed in sections below.

Many metrics are not given names to be used as scoring values, sometimes because they require additional parameters, such as fbeta_score. In such cases, you need to generate an appropriate scoring object. The simplest way to generate a callable object for scoring is by using make_scorer. That function converts metrics into callables that can be used for model evaluation.

One typical use case is to wrap an existing metric function from the library with non-default values for its parameters, such as the beta parameter for the fbeta_score function:

>>> from sklearn.metrics import fbeta_score, make_scorer
>>> ftwo_scorer = make_scorer(fbeta_score, beta=2)
>>> from sklearn.model_selection import GridSearchCV
>>> from sklearn.svm import LinearSVC
>>> grid = GridSearchCV(LinearSVC(), param_grid={‘C‘: [1, 10]}, scoring=ftwo_scorer)