Support vector machines (SVMs) are a set of supervised learning methods used for classification, regression and outliers detection.
The advantages of support vector machines are:
The disadvantages of support vector machines include:
The support vector machines in scikit-learn support both dense (numpy.ndarray
and convertible to that by numpy.asarray
) and sparse (any scipy.sparse
) sample vectors as input. However, to use an SVM to make predictions for sparse data, it must have been fit on such data. For optimal performance, use C-ordered numpy.ndarray
(dense) or scipy.sparse.csr_matrix
(sparse) with dtype=float64
.
SVC
, NuSVC
and LinearSVC
are classes capable of performing multi-class classification on a dataset.
SVC
and NuSVC
are similar methods, but accept slightly different sets of parameters and have different mathematical formulations (see section Mathematical formulation). On the other hand, LinearSVC
is another implementation of Support Vector Classification for the case of a linear kernel. Note that LinearSVC
does not accept keyword kernel
, as this is assumed to be linear. It also lacks some of the members of SVC
and NuSVC
, like support_
.
As other classifiers, SVC
, NuSVC
and LinearSVC
take as input two arrays: an array X of size [n_samples,
n_features]
holding the training samples, and an array y of class labels (strings or integers), size [n_samples]
:
>>> from sklearn import svm >>> X = [[0, 0], [1, 1]] >>> y = [0, 1] >>> clf = svm.SVC() >>> clf.fit(X, y) SVC(C=1.0, cache_size=200, class_weight=None, coef0=0.0, decision_function_shape='ovr', degree=3, gamma='auto', kernel='rbf', max_iter=-1, probability=False, random_state=None, shrinking=True, tol=0.001, verbose=False)
After being fitted, the model can then be used to predict new values:
>>> clf.predict([[2., 2.]]) array([1])
SVMs decision function depends on some subset of the training data, called the support vectors. Some properties of these support vectors can be found in members support_vectors_
, support_
and n_support
:
>>> # get support vectors >>> clf.support_vectors_ array([[ 0., 0.], [ 1., 1.]]) >>> # get indices of support vectors >>> clf.support_ array([0, 1]...) >>> # get number of support vectors for each class >>> clf.n_support_ array([1, 1]...)
SVC
and NuSVC
implement the “one-against-one” approach (Knerr et al., 1990) for multi- class classification. If n_class
is the number of classes, then n_class * (n_class - 1) / 2
classifiers are constructed and each one trains data from two classes. To provide a consistent interface with other classifiers, the decision_function_shape
option allows to aggregate the results of the “one-against-one” classifiers to a decision function of shape (n_samples,
n_classes)
:
>>> X = [[0], [1], [2], [3]] >>> Y = [0, 1, 2, 3] >>> clf = svm.SVC(decision_function_shape='ovo') >>> clf.fit(X, Y) SVC(C=1.0, cache_size=200, class_weight=None, coef0=0.0, decision_function_shape='ovo', degree=3, gamma='auto', kernel='rbf', max_iter=-1, probability=False, random_state=None, shrinking=True, tol=0.001, verbose=False) >>> dec = clf.decision_function([[1]]) >>> dec.shape[1] # 4 classes: 4*3/2 = 6 6 >>> clf.decision_function_shape = "ovr" >>> dec = clf.decision_function([[1]]) >>> dec.shape[1] # 4 classes 4
On the other hand, LinearSVC
implements “one-vs-the-rest” multi-class strategy, thus training n_class models. If there are only two classes, only one model is trained:
>>> lin_clf = svm.LinearSVC() >>> lin_clf.fit(X, Y) LinearSVC(C=1.0, class_weight=None, dual=True, fit_intercept=True, intercept_scaling=1, loss='squared_hinge', max_iter=1000, multi_class='ovr', penalty='l2', random_state=None, tol=0.0001, verbose=0) >>> dec = lin_clf.decision_function([[1]]) >>> dec.shape[1] 4
See Mathematical formulation for a complete description of the decision function.
Note that the LinearSVC
also implements an alternative multi-class strategy, the so-called multi-class SVM formulated by Crammer and Singer, by using the option multi_class='crammer_singer'
. This method is consistent, which is not true for one-vs-rest classification. In practice, one-vs-rest classification is usually preferred, since the results are mostly similar, but the runtime is significantly less.
For “one-vs-rest” LinearSVC
the attributes coef_
and intercept_
have the shape [n_class, n_features]
and [n_class]
respectively. Each row of the coefficients corresponds to one of the n_class
many “one-vs-rest” classifiers and similar for the intercepts, in the order of the “one” class.
In the case of “one-vs-one” SVC
, the layout of the attributes is a little more involved. In the case of having a linear kernel, The layout of coef_
and intercept_
is similar to the one described for LinearSVC
described above, except that the shape of coef_
is [n_class * (n_class - 1) / 2, n_features]
, corresponding to as many binary classifiers. The order for classes 0 to n is “0 vs 1”, “0 vs 2” , … “0 vs n”, “1 vs 2”, “1 vs 3”, “1 vs n”, . . . “n-1 vs n”.
The shape of dual_coef_
is [n_class-1, n_SV]
with a somewhat hard to grasp layout. The columns correspond to the support vectors involved in any of the n_class * (n_class - 1) / 2
“one-vs-one” classifiers. Each of the support vectors is used in n_class - 1
classifiers. The n_class - 1
entries in each row correspond to the dual coefficients for these classifiers.
This might be made more clear by an example:
Consider a three class problem with class 0 having three support vectors and class 1 and 2 having two support vectors and respectively. For each support vector , there are two dual coefficients. Let’s call the coefficient of support vector in the classifier between classes and . Then dual_coef_
looks like this:
Coefficients for SVs of class 0 | ||
Coefficients for SVs of class 1 | ||
Coefficients for SVs of class 2 | ||
The SVC
method decision_function
gives per-class scores for each sample (or a single score per sample in the binary case). When the constructor option probability
is set to True
, class membership probability estimates (from the methods predict_proba
and predict_log_proba
) are enabled. In the binary case, the probabilities are calibrated using Platt scaling: logistic regression on the SVM’s scores, fit by an additional cross-validation on the training data. In the multiclass case, this is extended as per Wu et al. (2004).
Needless to say, the cross-validation involved in Platt scaling is an expensive operation for large datasets. In addition, the probability estimates may be inconsistent with the scores, in the sense that the “argmax” of the scores may not be the argmax of the probabilities. (E.g., in binary classification, a sample may be labeled by predict
as belonging to a class that has probability <½ according to predict_proba
.) Platt’s method is also known to have theoretical issues. If confidence scores are required, but these do not have to be probabilities, then it is advisable to set probability=False
and use decision_function
instead of predict_proba
.
References:
“Probabilistic outputs for SVMs and comparisons to regularized likelihood methods” <http://www.cs.colorado.edu/~mozer/Teaching/syllabi/6622/papers/Platt1999.pdf>
.In problems where it is desired to give more importance to certain classes or certain individual samples keywords class_weight
and sample_weight
can be used.
SVC
(but not NuSVC
) implement a keyword class_weight
in the fit
method. It’s a dictionary of the form {class_label : value}
, where value is a floating point number > 0 that sets the parameter C
of class class_label
to C * value
.
SVC
, NuSVC
, SVR
, NuSVR
and OneClassSVM
implement also weights for individual samples in method fit
through keyword sample_weight
. Similar to class_weight
, these set the parameter C
for the i-th example to C * sample_weight[i]
.
Examples:
The method of Support Vector Classification can be extended to solve regression problems. This method is called Support Vector Regression.
The model produced by support vector classification (as described above) depends only on a subset of the training data, because the cost function for building the model does not care about training points that lie beyond the margin. Analogously, the model produced by Support Vector Regression depends only on a subset of the training data, because the cost function for building the model ignores any training data close to the model prediction.
There are three different implementations of Support Vector Regression: SVR
, NuSVR
and LinearSVR
. LinearSVR
provides a faster implementation than SVR
but only considers linear kernels, while NuSVR
implements a slightly different formulation than SVR
and LinearSVR
. See Implementation details for further details.
As with classification classes, the fit method will take as argument vectors X, y, only that in this case y is expected to have floating point values instead of integer values:
>>> from sklearn import svm >>> X = [[0, 0], [2, 2]] >>> y = [0.5, 2.5] >>> clf = svm.SVR() >>> clf.fit(X, y) SVR(C=1.0, cache_size=200, coef0=0.0, degree=3, epsilon=0.1, gamma='auto', kernel='rbf', max_iter=-1, shrinking=True, tol=0.001, verbose=False) >>> clf.predict([[1, 1]]) array([ 1.5])
One-class SVM is used for novelty detection, that is, given a set of samples, it will detect the soft boundary of that set so as to classify new points as belonging to that set or not. The class that implements this is called OneClassSVM
.
In this case, as it is a type of unsupervised learning, the fit method will only take as input an array X, as there are no class labels.
See, section Novelty and Outlier Detection for more details on this usage.
Support Vector Machines are powerful tools, but their compute and storage requirements increase rapidly with the number of training vectors. The core of an SVM is a quadratic programming problem (QP), separating support vectors from the rest of the training data. The QP solver used by this libsvm-based implementation scales between and depending on how efficiently the libsvm cache is used in practice (dataset dependent). If the data is very sparse should be replaced by the average number of non-zero features in a sample vector.
Also note that for the linear case, the algorithm used in LinearSVC
by the liblinear implementation is much more efficient than its libsvm-based SVC
counterpart and can scale almost linearly to millions of samples and/or features.
Avoiding data copy: For SVC
, SVR
, NuSVC
and NuSVR
, if the data passed to certain methods is not C-ordered contiguous, and double precision, it will be copied before calling the underlying C implementation. You can check whether a given numpy array is C-contiguous by inspecting its flags
attribute.
For LinearSVC
(and LogisticRegression
) any input passed as a numpy array will be copied and converted to the liblinear internal sparse data representation (double precision floats and int32 indices of non-zero components). If you want to fit a large-scale linear classifier without copying a dense numpy C-contiguous double precision array as input we suggest to use the SGDClassifier
class instead. The objective function can be configured to be almost the same as the LinearSVC
model.
SVC
, SVR
, nuSVC
and NuSVR
, the size of the kernel cache has a strong impact on run times for larger problems. If you have enough RAM available, it is recommended to set cache_size
to a higher value than the default of 200(MB), such as 500(MB) or 1000(MB). C
is 1
by default and it’s a reasonable default choice. If you have a lot of noisy observations you should decrease it. It corresponds to regularize more the estimation. nu
in NuSVC
/OneClassSVM
/NuSVR
approximates the fraction of training errors and support vectors. SVC
, if data for classification are unbalanced (e.g. many positive and few negative), set class_weight='balanced'
and/or try different penalty parameters C
. LinearSVC
implementation uses a random number generator to select features when fitting the model. It is thus not uncommon, to have slightly different results for the same input data. If that happens, try with a smaller tol parameter. LinearSVC(loss='l2', penalty='l1',
dual=False)
yields a sparse solution, i.e. only a subset of feature weights is different from zero and contribute to the decision function. Increasing C
yields a more complex model (more feature are selected). The C
value that yields a “null” model (all weights equal to zero) can be calculated using l1_min_c
. The kernel function can be any of the following:
degree
, by coef0
.gamma
, must be greater than 0.coef0
.Different kernels are specified by keyword kernel at initialization:
>>> linear_svc = svm.SVC(kernel='linear') >>> linear_svc.kernel 'linear' >>> rbf_svc = svm.SVC(kernel='rbf') >>> rbf_svc.kernel 'rbf'
You can define your own kernels by either giving the kernel as a python function or by precomputing the Gram matrix.
Classifiers with custom kernels behave the same way as any other classifiers, except that:
support_vectors_
is now empty, only indices of support vectors are stored in support_
fit()
method is stored for future reference. If that array changes between the use of fit()
and predict()
you will have unexpected results.You can also use your own defined kernels by passing a function to the keyword kernel
in the constructor.
Your kernel must take as arguments two matrices of shape (n_samples_1, n_features)
, (n_samples_2, n_features)
and return a kernel matrix of shape (n_samples_1, n_samples_2)
.
The following code defines a linear kernel and creates a classifier instance that will use that kernel:
>>> import numpy as np >>> from sklearn import svm >>> def my_kernel(X, Y): ... return np.dot(X, Y.T) ... >>> clf = svm.SVC(kernel=my_kernel)
Examples:
Set kernel='precomputed'
and pass the Gram matrix instead of X in the fit method. At the moment, the kernel values between all training vectors and the test vectors must be provided.
>>> import numpy as np >>> from sklearn import svm >>> X = np.array([[0, 0], [1, 1]]) >>> y = [0, 1] >>> clf = svm.SVC(kernel='precomputed') >>> # linear kernel computation >>> gram = np.dot(X, X.T) >>> clf.fit(gram, y) SVC(C=1.0, cache_size=200, class_weight=None, coef0=0.0, decision_function_shape='ovr', degree=3, gamma='auto', kernel='precomputed', max_iter=-1, probability=False, random_state=None, shrinking=True, tol=0.001, verbose=False) >>> # predict on training examples >>> clf.predict(gram) array([0, 1])
When training an SVM with the Radial Basis Function (RBF) kernel, two parameters must be considered: C
and gamma
. The parameter C
, common to all SVM kernels, trades off misclassification of training examples against simplicity of the decision surface. A low C
makes the decision surface smooth, while a high C
aims at classifying all training examples correctly. gamma
defines how much influence a single training example has. The larger gamma
is, the closer other examples must be to be affected.
Proper choice of C
and gamma
is critical to the SVM’s performance. One is advised to use sklearn.model_selection.GridSearchCV
with C
and gamma
spaced exponentially far apart to choose good values.
Examples:
A support vector machine constructs a hyper-plane or set of hyper-planes in a high or infinite dimensional space, which can be used for classification, regression or other tasks. Intuitively, a good separation is achieved by the hyper-plane that has the largest distance to the nearest training data points of any class (so-called functional margin), since in general the larger the margin the lower the generalization error of the classifier.
Given training vectors , i=1,…, n, in two classes, and a vector , SVC solves the following primal problem:
Its dual is
where is the vector of all ones, is the upper bound, is an by positive semidefinite matrix, , where is the kernel. Here training vectors are implicitly mapped into a higher (maybe infinite) dimensional space by the function .
The decision function is:
Note
While SVM models derived from libsvm and liblinear use C
as regularization parameter, most other estimators use alpha
. The exact equivalence between the amount of regularization of two models depends on the exact objective function optimized by the model. For example, when the estimator used is sklearn.linear_model.Ridge
regression, the relation between them is given as .
This parameters can be accessed through the members dual_coef_
which holds the product , support_vectors_
which holds the support vectors, and intercept_
which holds the independent term :
References:
We introduce a new parameter which controls the number of support vectors and training errors. The parameter is an upper bound on the fraction of training errors and a lower bound of the fraction of support vectors.
It can be shown that the -SVC formulation is a reparameterization of the -SVC and therefore mathematically equivalent.
Given training vectors , i=1,…, n, and a vector -SVR solves the following primal problem:
Its dual is
where is the vector of all ones, is the upper bound, is an by positive semidefinite matrix, is the kernel. Here training vectors are implicitly mapped into a higher (maybe infinite) dimensional space by the function .
The decision function is:
These parameters can be accessed through the members dual_coef_
which holds the difference , support_vectors_
which holds the support vectors, and intercept_
which holds the independent term
References:
Internally, we use libsvm and liblinear to handle all computations. These libraries are wrapped using C and Cython.
References:
For a description of the implementation and details of the algorithms used, please refer to
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Licensed under the 3-clause BSD License.
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