1.12. Multiclass and multilabel algorithms¶
Warning
All classifiers in scikitlearn do multiclass classification
outofthebox. You don’t need to use the sklearn.multiclass
module
unless you want to experiment with different multiclass strategies.
The sklearn.multiclass
module implements metaestimators to solve
multiclass
and multilabel
classification problems
by decomposing such problems into binary classification problems. multioutput
regression is also supported.
Multiclass classification: classification task with more than two classes. Each sample can only be labelled as one class.
For example, classification using features extracted from a set of images of fruit, where each image may either be of an orange, an apple, or a pear. Each image is one sample and is labelled as one of the 3 possible classes. Multiclass classification makes the assumption that each sample is assigned to one and only one label  one sample cannot, for example, be both a pear and an apple.
Valid multiclass representations for
type_of_target
(y
) are:1d or column vector containing more than two discrete values. An example of a vector
y
for 3 samples:>>> import numpy as np >>> y = np.array(['apple', 'pear', 'apple']) >>> print(y) ['apple' 'pear' 'apple']
sparse binary matrix of shape
(n_samples, n_classes)
with a single element per row, where each column represents one class. An example of a sparse binary matrixy
for 3 samples, where the columns, in order, are orange, apple and pear:>>> from scipy import sparse >>> row_ind = np.array([0, 1, 2]) >>> col_ind = np.array([1, 2, 1]) >>> y_sparse = sparse.csr_matrix((np.ones(3), (row_ind, col_ind))) >>> print(y_sparse) (0, 1) 1.0 (1, 2) 1.0 (2, 1) 1.0
Multilabel classification: classification task labelling each sample with
x
labels fromn_classes
possible classes, wherex
can be 0 ton_classes
inclusive. This can be thought of as predicting properties of a sample that are not mutually exclusive. Formally, a binary output is assigned to each class, for every sample. Positive classes are indicated with 1 and negative classes with 0 or 1. It is thus comparable to runningn_classes
binary classification tasks, for example withsklearn.multioutput.MultiOutputClassifier
. This approach treats each label independently whereas multilabel classifiers may treat the multiple classes simultaneously, accounting for correlated behaviour amoung them.For example, prediction of the topics relevant to a text document or video. The document or video may be about one of ‘religion’, ‘politics’, ‘finance’ or ‘education’, several of the topic classes or all of the topic classes.
Valid representation of multilabel
y
is either dense (or sparse) binary matrix of shape(n_samples, n_classes)
. Each column represents a class. The1
’s in each row denote the positive classes a sample has been labelled with. An example of a dense matrixy
for 3 samples:>>> y = np.array([[1, 0, 0, 1], [0, 0, 1, 1], [0, 0, 0, 0]]) >>> print(y) [[1 0 0 1] [0 0 1 1] [0 0 0 0]]
An example of the same
y
in sparse matrix form:>>> y_sparse = sparse.csr_matrix(y) >>> print(y_sparse) (0, 0) 1 (0, 3) 1 (1, 2) 1 (1, 3) 1
Multioutput regression: predicts multiple numerical properties for each sample. Each property is a numerical variable and the number of properties to be predicted for each sample is greater than or equal to 2. Some estimators that support multioutput regression are faster than just running
n_output
estimators.For example, prediction of both wind speed and wind direction, in degrees, using data obtained at a certain location. Each sample would be data obtained at one location and both wind speed and directtion would be output for each sample.
Valid representation of multilabel
y
is dense matrix of shape(n_samples, n_classes)
of floats. A column wise concatenation of continuous variables. An example ofy
for 3 samples:>>> y = np.array([[31.4, 94], [40.5, 109], [25.0, 30]]) >>> print(y) [[ 31.4 94. ] [ 40.5 109. ] [ 25. 30. ]]
Multioutputmulticlass classification (also known as multitask classification): classification task which labels each sample with a set of nonbinary properties. Both the number of properties and the number of classes per property is greater than 2. A single estimator thus handles several joint classification tasks. This is both a generalization of the multilabel classification task, which only considers binary attributes, as well as a generalization of the multiclass classification task, where only one property is considered.
For example, classification of the properties “type of fruit” and “colour” for a set of images of fruit. The property “type of fruit” has the possible classes: “apple”, “pear” and “orange”. The property “colour” has the possible classes: “green”, “red”, “yellow” and “orange”. Each sample is an image of a fruit, a label is output for both properties and each label is one of the possible classes of the corresponding property.
Valid representation of multilabel
y
is dense matrix of shape(n_samples, n_classes)
of floats. A column wise concatenation of 1d multiclass variables. An example ofy
for 3 samples:>>> y = np.array([['apple', 'green'], ['orange', 'orange'], ['pear', 'green']]) >>> print(y) [['apple' 'green'] ['orange' 'orange'] ['pear' 'green']]
Note that any classifiers handling multioutputmulticlass (also known as multitask classification) tasks, support the multilabel classification task as a special case. Multitask classification is similar to the multioutput classification task with different model formulations. For more information, see the relevant estimator documentation.
All scikitlearn classifiers are capable of multiclass classification,
but the metaestimators offered by sklearn.multiclass
permit changing the way they handle more than two classes
because this may have an effect on classifier performance
(either in terms of generalization error or required computational resources).
Summary
Number of targets 
Target cardinality 
Valid



Multiclass classification 
1 
>2 

Multilabel classification 
>1 
2 (0 or 1) 

Multioutput regression 
>1 
Continuous 

Multioutput multiclass classification 
>1 
>2 

Below is a summary of the classifiers supported by scikitlearn grouped by strategy; you don’t need the metaestimators in this class if you’re using one of these, unless you want custom multiclass behavior:
Inherently multiclass:
sklearn.svm.LinearSVC
(setting multi_class=”crammer_singer”)sklearn.linear_model.LogisticRegression
(setting multi_class=”multinomial”)sklearn.linear_model.LogisticRegressionCV
(setting multi_class=”multinomial”)
Multiclass as OneVsOne:
sklearn.gaussian_process.GaussianProcessClassifier
(setting multi_class = “one_vs_one”)
Multiclass as OneVsTheRest:
sklearn.gaussian_process.GaussianProcessClassifier
(setting multi_class = “one_vs_rest”)sklearn.svm.LinearSVC
(setting multi_class=”ovr”)sklearn.linear_model.LogisticRegression
(setting multi_class=”ovr”)sklearn.linear_model.LogisticRegressionCV
(setting multi_class=”ovr”)
Support multilabel:
Support multiclassmultioutput:
Warning
At present, no metric in sklearn.metrics
supports the multioutputmulticlass classification task.
1.12.1. Multilabel classification format¶
In multilabel learning, the joint set of binary classification tasks is
expressed with label binary indicator array: each sample is one row of a 2d
array of shape (n_samples, n_classes) with binary values: the one, i.e. the non
zero elements, corresponds to the subset of labels. An array such as
np.array([[1, 0, 0], [0, 1, 1], [0, 0, 0]])
represents label 0 in the first
sample, labels 1 and 2 in the second sample, and no labels in the third sample.
Producing multilabel data as a list of sets of labels may be more intuitive.
The MultiLabelBinarizer
transformer can be used to convert between a collection of collections of
labels and the indicator format.
>>> from sklearn.preprocessing import MultiLabelBinarizer
>>> y = [[2, 3, 4], [2], [0, 1, 3], [0, 1, 2, 3, 4], [0, 1, 2]]
>>> MultiLabelBinarizer().fit_transform(y)
array([[0, 0, 1, 1, 1],
[0, 0, 1, 0, 0],
[1, 1, 0, 1, 0],
[1, 1, 1, 1, 1],
[1, 1, 1, 0, 0]])
1.12.2. OneVsTheRest¶
This strategy, also known as onevsall, is implemented in
OneVsRestClassifier
. The strategy consists in fitting one classifier
per class. For each classifier, the class is fitted against all the other
classes. In addition to its computational efficiency (only n_classes
classifiers are needed), one advantage of this approach is its
interpretability. Since each class is represented by one and only one classifier,
it is possible to gain knowledge about the class by inspecting its
corresponding classifier. This is the most commonly used strategy and is a fair
default choice.
1.12.2.1. Multiclass learning¶
Below is an example of multiclass learning using OvR:
>>> from sklearn import datasets
>>> from sklearn.multiclass import OneVsRestClassifier
>>> from sklearn.svm import LinearSVC
>>> X, y = datasets.load_iris(return_X_y=True)
>>> OneVsRestClassifier(LinearSVC(random_state=0)).fit(X, y).predict(X)
array([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 1, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2])
1.12.2.2. Multilabel learning¶
OneVsRestClassifier
also supports multilabel classification.
To use this feature, feed the classifier an indicator matrix, in which cell
[i, j] indicates the presence of label j in sample i.
Examples:
1.12.3. OneVsOne¶
OneVsOneClassifier
constructs one classifier per pair of classes.
At prediction time, the class which received the most votes is selected.
In the event of a tie (among two classes with an equal number of votes), it
selects the class with the highest aggregate classification confidence by
summing over the pairwise classification confidence levels computed by the
underlying binary classifiers.
Since it requires to fit n_classes * (n_classes  1) / 2
classifiers,
this method is usually slower than onevstherest, due to its
O(n_classes^2) complexity. However, this method may be advantageous for
algorithms such as kernel algorithms which don’t scale well with
n_samples
. This is because each individual learning problem only involves
a small subset of the data whereas, with onevstherest, the complete
dataset is used n_classes
times. The decision function is the result
of a monotonic transformation of the oneversusone classification.
1.12.3.1. Multiclass learning¶
Below is an example of multiclass learning using OvO:
>>> from sklearn import datasets
>>> from sklearn.multiclass import OneVsOneClassifier
>>> from sklearn.svm import LinearSVC
>>> X, y = datasets.load_iris(return_X_y=True)
>>> OneVsOneClassifier(LinearSVC(random_state=0)).fit(X, y).predict(X)
array([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 2, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2])
References:
“Pattern Recognition and Machine Learning. Springer”, Christopher M. Bishop, page 183, (First Edition)
1.12.4. ErrorCorrecting OutputCodes¶
Outputcode based strategies are fairly different from onevstherest and onevsone. With these strategies, each class is represented in a Euclidean space, where each dimension can only be 0 or 1. Another way to put it is that each class is represented by a binary code (an array of 0 and 1). The matrix which keeps track of the location/code of each class is called the code book. The code size is the dimensionality of the aforementioned space. Intuitively, each class should be represented by a code as unique as possible and a good code book should be designed to optimize classification accuracy. In this implementation, we simply use a randomlygenerated code book as advocated in 3 although more elaborate methods may be added in the future.
At fitting time, one binary classifier per bit in the code book is fitted. At prediction time, the classifiers are used to project new points in the class space and the class closest to the points is chosen.
In OutputCodeClassifier
, the code_size
attribute allows the user to
control the number of classifiers which will be used. It is a percentage of the
total number of classes.
A number between 0 and 1 will require fewer classifiers than
onevstherest. In theory, log2(n_classes) / n_classes
is sufficient to
represent each class unambiguously. However, in practice, it may not lead to
good accuracy since log2(n_classes)
is much smaller than n_classes.
A number greater than 1 will require more classifiers than onevstherest. In this case, some classifiers will in theory correct for the mistakes made by other classifiers, hence the name “errorcorrecting”. In practice, however, this may not happen as classifier mistakes will typically be correlated. The errorcorrecting output codes have a similar effect to bagging.
1.12.4.1. Multiclass learning¶
Below is an example of multiclass learning using OutputCodes:
>>> from sklearn import datasets
>>> from sklearn.multiclass import OutputCodeClassifier
>>> from sklearn.svm import LinearSVC
>>> X, y = datasets.load_iris(return_X_y=True)
>>> clf = OutputCodeClassifier(LinearSVC(random_state=0),
... code_size=2, random_state=0)
>>> clf.fit(X, y).predict(X)
array([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1,
1, 2, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 1, 1, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2])
References:
“Solving multiclass learning problems via errorcorrecting output codes”, Dietterich T., Bakiri G., Journal of Artificial Intelligence Research 2, 1995.
 3
“The error coding method and PICTs”, James G., Hastie T., Journal of Computational and Graphical statistics 7, 1998.
“The Elements of Statistical Learning”, Hastie T., Tibshirani R., Friedman J., page 606 (secondedition) 2008.
1.12.5. Multioutput regression¶
Multioutput regression support can be added to any regressor with
MultiOutputRegressor
. This strategy consists of fitting one
regressor per target. Since each target is represented by exactly one
regressor it is possible to gain knowledge about the target by
inspecting its corresponding regressor. As
MultiOutputRegressor
fits one regressor per target it can not
take advantage of correlations between targets.
Below is an example of multioutput regression:
>>> from sklearn.datasets import make_regression
>>> from sklearn.multioutput import MultiOutputRegressor
>>> from sklearn.ensemble import GradientBoostingRegressor
>>> X, y = make_regression(n_samples=10, n_targets=3, random_state=1)
>>> MultiOutputRegressor(GradientBoostingRegressor(random_state=0)).fit(X, y).predict(X)
array([[154.75474165, 147.03498585, 50.03812219],
[ 7.12165031, 5.12914884, 81.46081961],
[187.8948621 , 100.44373091, 13.88978285],
[141.62745778, 95.02891072, 191.48204257],
[ 97.03260883, 165.34867495, 139.52003279],
[ 123.92529176, 21.25719016, 7.84253 ],
[122.25193977, 85.16443186, 107.12274212],
[ 30.170388 , 94.80956739, 12.16979946],
[ 140.72667194, 176.50941682, 17.50447799],
[ 149.37967282, 81.15699552, 5.72850319]])
1.12.6. Multioutput classification¶
Multioutput classification support can be added to any classifier with
MultiOutputClassifier
. This strategy consists of fitting one
classifier per target. This allows multiple target variable
classifications. The purpose of this class is to extend estimators
to be able to estimate a series of target functions (f1,f2,f3…,fn)
that are trained on a single X predictor matrix to predict a series
of responses (y1,y2,y3…,yn).
Below is an example of multioutput classification:
>>> from sklearn.datasets import make_classification
>>> from sklearn.multioutput import MultiOutputClassifier
>>> from sklearn.ensemble import RandomForestClassifier
>>> from sklearn.utils import shuffle
>>> import numpy as np
>>> X, y1 = make_classification(n_samples=10, n_features=100, n_informative=30, n_classes=3, random_state=1)
>>> y2 = shuffle(y1, random_state=1)
>>> y3 = shuffle(y1, random_state=2)
>>> Y = np.vstack((y1, y2, y3)).T
>>> n_samples, n_features = X.shape # 10,100
>>> n_outputs = Y.shape[1] # 3
>>> n_classes = 3
>>> forest = RandomForestClassifier(random_state=1)
>>> multi_target_forest = MultiOutputClassifier(forest, n_jobs=1)
>>> multi_target_forest.fit(X, Y).predict(X)
array([[2, 2, 0],
[1, 2, 1],
[2, 1, 0],
[0, 0, 2],
[0, 2, 1],
[0, 0, 2],
[1, 1, 0],
[1, 1, 1],
[0, 0, 2],
[2, 0, 0]])
1.12.7. Classifier Chain¶
Classifier chains (see ClassifierChain
) are a way of combining a
number of binary classifiers into a single multilabel model that is capable
of exploiting correlations among targets.
For a multilabel classification problem with N classes, N binary classifiers are assigned an integer between 0 and N1. These integers define the order of models in the chain. Each classifier is then fit on the available training data plus the true labels of the classes whose models were assigned a lower number.
When predicting, the true labels will not be available. Instead the predictions of each model are passed on to the subsequent models in the chain to be used as features.
Clearly the order of the chain is important. The first model in the chain has no information about the other labels while the last model in the chain has features indicating the presence of all of the other labels. In general one does not know the optimal ordering of the models in the chain so typically many randomly ordered chains are fit and their predictions are averaged together.
References:
 Jesse Read, Bernhard Pfahringer, Geoff Holmes, Eibe Frank,
“Classifier Chains for Multilabel Classification”, 2009.
1.12.8. Regressor Chain¶
Regressor chains (see RegressorChain
) is analogous to
ClassifierChain as a way of combining a number of regressions
into a single multitarget model that is capable of exploiting
correlations among targets.