
.. _sgd:

===========================
Stochastic Gradient Descent
===========================

.. currentmodule:: scikits.learn.linear_model

**Stochastic Gradient Descent (SGD)** is a simple yet very efficient
approach to discriminative learning of linear classifiers under
convex loss functions such as (linear) `Support Vector Machines
<http://en.wikipedia.org/wiki/Support_vector_machine>`_ and `Logistic
Regression <http://en.wikipedia.org/wiki/Logistic_regression>`_.
Even though SGD has been around in the machine learning community for
a long time, it has received a considerable amount of attention just
recently in the context of large-scale learning.

SGD has been successfully applied to large-scale and sparse machine
learning problems often encountered in text classification and natural
language processing.  Given that the data is sparse, the classifiers
in this module easily scale to problems with more than 10^5 training
examples and more than 10^4 features.

The advantages of Stochastic Gradient Descent are:

    - Efficiency.

    - Ease of implementation (lots of opportunities for code tuning).

The disadvantages of Stochastic Gradient Descent include:

    - SGD requires a number of hyperparameters including the regularization
      parameter and the number of iterations.

    - SGD is sensitive to feature scaling.

Classification
==============

.. warning:: Make sure you permute (shuffle) your training data before fitting the model or use `shuffle=True` to shuffle after each iterations.

The class :class:`SGDClassifier` implements a plain stochastic gradient
descent learning routine which supports different loss functions and
penalties for classification.

.. figure:: ../auto_examples/linear_model/images/plot_sgd_separating_hyperplane.png
   :target: ../auto_examples/linear_model/plot_sgd_separating_hyperplane.html
   :align: center
   :scale: 75

As other classifiers, SGD has to be fitted with two arrays: an array X
of size [n_samples, n_features] holding the training samples, and an
array Y of size [n_samples] holding the target values (class labels)
for the training samples::

    >>> from scikits.learn.linear_model import SGDClassifier
    >>> X = [[0., 0.], [1., 1.]]
    >>> y = [0, 1]
    >>> clf = SGDClassifier(loss="hinge", penalty="l2")
    >>> clf.fit(X, y)
    SGDClassifier(loss='hinge', n_jobs=1, shuffle=False, verbose=0, n_iter=5,
           fit_intercept=True, penalty='l2', rho=1.0, alpha=0.0001)

After being fitted, the model can then be used to predict new values::

    >>> clf.predict([[2., 2.]])
    array([ 1.])

SGD fits a linear model to the training data. The member `coef_` holds
the model parameters::

    >>> clf.coef_
    array([ 9.90090187,  9.90090187])

Member `intercept_` holds the intercept (aka offset or bias)::

    >>> clf.intercept_
    array(-9.9900299301496904)

Whether or not the model should use an intercept, i.e. a biased
hyperplane, is controlled by the parameter `fit_intercept`.

To get the signed distance to the hyperplane use `decision_function`::

    >>> clf.decision_function([[2., 2.]])
    array([ 29.61357756])

The concrete loss function can be set via the `loss`
parameter. :class:`SGDClassifier` supports the following loss functions:

  - `loss="hinge"`: (soft-margin) linear Support Vector Machine.
  - `loss="modified_huber"`: smoothed hinge loss.
  - `loss="log"`: Logistic Regression

The first two loss functions are lazy, they only update the model
parameters if an example violates the margin constraint, which makes
training very efficient.  Log loss, on the other hand, provides
probability estimates.

In the case of binary classification and `loss="log"` you get a
probability estimate P(y=C|x) using `predict_proba`, where `C` is the
largest class label::

    >>> clf = SGDClassifier(loss="log").fit(X, y)
    >>> clf.predict_proba([[1., 1.]])
    array([ 0.99999949])

The concrete penalty can be set via the `penalty` parameter. `SGD`
supports the following penalties:

  - `penalty="l2"`: L2 norm penalty on `coef_`.
  - `penalty="l1"`: L1 norm penalty on `coef_`.
  - `penalty="elasticnet"`: Convex combination of L2 and L1; `rho * L2 + (1 - rho) * L1`.

The default setting is `penalty="l2"`. The L1 penalty leads to sparse
solutions, driving most coefficients to zero. The Elastic Net solves
some deficiencies of the L1 penalty in the presence of highly correlated
attributes. The parameter `rho` has to be specified by the user.

:class:`SGDClassifier` supports multi-class classification by combining
multiple binary classifiers in a "one versus all" (OVA) scheme. For each
of the `K` classes, a binary classifier is learned that discriminates
between that and all other `K-1` classes. At testing time, we compute the
confidence score (i.e. the signed distances to the hyperplane) for each
classifier and choose the class with the highest confidence. The Figure
below illustrates the OVA approach on the iris dataset.  The dashed
lines represent the three OVA classifiers; the background colors show
the decision surface induced by the three classifiers.

.. figure:: ../auto_examples/linear_model/images/plot_sgd_iris.png
   :target: ../auto_examples/linear_model/plot_sgd_iris.html
   :align: center
   :scale: 75

In the case of multi-class classification `coef_` is a two-dimensionaly
array of shape [n_classes, n_features] and `intercept_` is a one
dimensional array of shape [n_classes]. The i-th row of `coef_` holds
the weight vector of the OVA classifier for the i-th class; classes are
indexed in ascending order (see member `classes`).

.. topic:: Examples:

 * :ref:`example_linear_model_plot_sgd_separating_hyperplane.py`,
 * :ref:`example_linear_model_plot_sgd_iris.py`

Regression
==========

The class :class:`SGDRegressor` implements a plain stochastic gradient
descent learning routine which supports different loss functions and
penalties to fit linear regression models.

.. figure:: ../auto_examples/linear_model/images/plot_sgd_ols.png
   :target: ../auto_examples/linear_model/plot_sgd_ols.html
   :align: center
   :scale: 75

The concrete loss function can be set via the `loss`
parameter. :class:`SGDRegressor` supports the following loss functions:

  - `loss="squared_loss"`: Ordinary least squares.
  - `loss="huber"`: Huber loss for robust regression.

.. topic:: Examples:

 * :ref:`example_linear_model_plot_sgd_ols.py`,


.. currentmodule:: scikits.learn.linear_model.sparse

Stochastic Gradient Descent for sparse data
===========================================

.. note:: The sparse implementation produces slightly different results
  than the dense implementation due to a shrunk learning rate for the
  intercept.

There is support for sparse data given in any matrix in a format
supported by scipy.sparse. Classes have the same name, just prefixed
by the `sparse` namespace, and take the same arguments, with the
exception of training and test data, which is expected to be in a
matrix format defined in scipy.sparse.

For maximum efficiency, use the CSR matrix format as defined in
`scipy.sparse.csr_matrix
<http://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csr_matrix.html>`_.

Implemented classes are :class:`SGDClassifier` and :class:`SGDRegressor`.

.. topic:: Examples:

 * :ref:`example_document_classification_20newsgroups.py`

Complexity
==========

The major advantage of SGD is its efficiency, which is basically
linear in the number of training examples. If X is a matrix of size (n, p)
training has a cost of :math:`O(k n \bar p)`, where k is the number
of iterations (epochs) and :math:`\bar p` is the average number of
non-zero attributes per sample.

Recent theoretical results, however, show that the runtime to get some
desired optimization accuracy does not increase as the training set size increases.

Tips on Practical Use
=====================

  * Stochastic Gradient Descent is sensitive to feature scaling, so it
    is highly recommended to scale your data. For example, scale each
    attribute on the input vector X to [0,1] or [-1,+1], or standardize
    it to have mean 0 and variance 1. Note that the *same* scaling
    must be applied to the test vector to obtain meaningful
    results. See `The CookBook
    <https://sourceforge.net/apps/trac/scikit-learn/wiki/CookBook>`_
    for some examples on scaling. If your attributes have an intrinsic
    scale (e.g. word frequencies or indicator features) scaling is
    not needed.

  * Finding a reasonable regularization term :math:`\alpha` is
    best done using grid search `for alpha in 10.0**-np.arange(1,7)`.

  * Empirically, we found that SGD converges after observing
    approx. 10^6 training samples. Thus, a reasonable first guess
    for the number of iterations is `n_iter = np.ceil(10**6 / n)`,
    where `n` is the size of the training set.

.. topic:: References:

 * `"Efficient BackProp" <yann.lecun.com/exdb/publis/pdf/lecun-98b.pdf>`_
   Y. LeCun, L. Bottou, G. Orr, K. Müller - In Neural Networks: Tricks
   of the Trade 1998.

.. _sgd_mathematical_formulation:

Mathematical formulation
========================

Given a set of training examples :math:`(x_1, y_1), \ldots, (x_n, y_n)` where
:math:`x_i \in \mathbf{R}^n` and :math:`y_i \in \{-1,1\}`, our goal is to
learn a linear scoring function :math:`f(x) = w^T x + b` with model parameters
:math:`w \in \mathbf{R}^m` and intercept :math:`b \in \mathbf{R}`. In order
to make predictions, we simply look at the sign of :math:`f(x)`.
A common choice to find the model parameters is by minimizing the regularized
training error given by

.. math::

    E(w,b) = \sum_{i=1}^{n} L(y_i, f(x_i)) + \alpha R(w)

where :math:`L` is a loss function that measures model (mis)fit and
:math:`R` is a regularization term (aka penalty) that penalizes model
complexity; :math:`\alpha > 0` is a non-negative hyperparameter.

Different choices for :math:`L` entail different classifiers such as

   - Hinge: (soft-margin) Support Vector Machines.
   - Log:   Logistic Regression.
   - Least-Squares: Ridge Regression.

All of the above loss functions can be regarded as an upper bound on the
misclassification error (Zero-one loss) as shown in the Figure below.

.. figure:: ../auto_examples/linear_model/images/plot_sgd_loss_functions.png
   :align: center
   :scale: 75

Popular choices for the regularization term :math:`R` include:

   - L2 norm: :math:`R(w) := \frac{1}{2} \sum_{i=1}^{n} w_i^2`,
   - L1 norm: :math:`R(w) := \sum_{i=1}^{n} |w_i|`, which leads to sparse
     solutions.
   - Elastic Net: :math:`R(w) := \rho \frac{1}{2} \sum_{i=1}^{n} w_i^2 + (1-\rho) \sum_{i=1}^{n} |w_i|`, a convex combination of L2 and L1.

The Figure below shows the contours of the different regularization terms
in the parameter space when :math:`R(w) = 1`.

.. figure:: ../auto_examples/linear_model/images/plot_sgd_penalties.png
   :align: center
   :scale: 75

SGD
---

Stochastic gradient descent is an optimization method for unconstrained
optimization problems. In contrast to (batch) gradient descent, SGD
approximates the true gradient of :math:`E(w,b)` by considering a
single training example at a time.

The class :class:`SGDClassifier` implements a first-order SGD learning
routine.  The algorithm iterates over the training examples and for each
example updates the model parameters according to the update rule given by

.. math::

    w \leftarrow w - \eta (\alpha \frac{\partial R(w)}{\partial w}
    + \frac{\partial L(w^T x_i + b, y_i)}{\partial w})

where :math:`\eta` is the learning rate which controls the step-size in
the parameter space.  The intercept :math:`b` is updated similarly but
without regularization.

The model parameters can be accessed through the members coef\_ and
intercept\_:

     - Member coef\_ holds the weights :math:`w`

     - Member intercept\_ holds :math:`b`

.. topic:: References:

 * `"Solving large scale linear prediction problems using stochastic
   gradient descent algorithms"
   <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.58.7377>`_
   T. Zhang - In Proceedings of ICML '04.

 * `"Regularization and variable selection via the elastic net"
   <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.124.4696>`_
   H. Zou, T. Hastie - Journal of the Royal Statistical Society Series B,
   67 (2), 301-320.


Implementation details
======================

The implementation of SGD is influenced by the `Stochastic Gradient SVM
<http://leon.bottou.org/projects/sgd>`_  of Léon Bottou. Similar to SvmSGD,
the weight vector is represented as the product of a scalar and a vector
which allows an efficient weight update in the case of L2 regularization.
In the case of sparse feature vectors, the intercept is updated with a
smaller learning rate (multiplied by 0.01) to account for the fact that
it is updated more frequently. Training examples are picked up sequentially
and the learning rate is lowered after each observed example. We adopted the
learning rate schedule from Shalev-Shwartz et al. 2007.
For multi-class classification, a "one versus all" approach is used.
We use the truncated gradient algorithm proposed by Tsuruoka et al. 2009
for L1 regularization (and the Elastic Net).
The code is written in Cython.

.. topic:: References:

 * `"Stochastic Gradient Descent" <http://leon.bottou.org/projects/sgd>`_ L. Bottou - Website, 2010.

 * `"Pegasos: Primal estimated sub-gradient solver for svm"
   <http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.74.8513>`_
   S. Shalev-Shwartz, Y. Singer, N. Srebro - In Proceedings of ICML '07.

 * `"Stochastic gradient descent training for l1-regularized log-linear models with cumulative penalty"
   <www.aclweb.org/anthology/P/P09/P09-1054.pdf>`_
   Y. Tsuruoka, J. Tsujii, S. Ananiadou -  In Proceedings of the AFNLP/ACL '09.
