class sklearn.cross_validation.StratifiedKFold(y, n_folds=3, shuffle=False, random_state=None)[source]

Stratified K-Folds cross validation iterator

Deprecated since version 0.18: This module will be removed in 0.20. Use sklearn.model_selection.StratifiedKFold instead.

Provides train/test indices to split data in train test sets.

This cross-validation object is a variation of KFold that returns stratified folds. The folds are made by preserving the percentage of samples for each class.

Read more in the User Guide.


y : array-like, [n_samples]

Samples to split in K folds.

n_folds : int, default=3

Number of folds. Must be at least 2.

shuffle : boolean, optional

Whether to shuffle each stratification of the data before splitting into batches.

random_state : int, RandomState instance or None, optional, default=None

If int, random_state is the seed used by the random number generator; If RandomState instance, random_state is the random number generator; If None, the random number generator is the RandomState instance used by np.random. Used when shuffle == True.

See also

K-fold iterator variant with non-overlapping labels.


All the folds have size trunc(n_samples / n_folds), the last one has the complementary.


>>> from sklearn.cross_validation import StratifiedKFold
>>> X = np.array([[1, 2], [3, 4], [1, 2], [3, 4]])
>>> y = np.array([0, 0, 1, 1])
>>> skf = StratifiedKFold(y, n_folds=2)
>>> len(skf)
>>> print(skf)  
sklearn.cross_validation.StratifiedKFold(labels=[0 0 1 1], n_folds=2,
                                         shuffle=False, random_state=None)
>>> for train_index, test_index in skf:
...    print("TRAIN:", train_index, "TEST:", test_index)
...    X_train, X_test = X[train_index], X[test_index]
...    y_train, y_test = y[train_index], y[test_index]
TRAIN: [1 3] TEST: [0 2]
TRAIN: [0 2] TEST: [1 3]
.. automethod:: __init__