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<title>GNU Scientific Library – Reference Manual: Robust linear regression</title>
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<a name="Robust-linear-regression"></a>
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Next: <a href="Troubleshooting.html#Troubleshooting" accesskey="n" rel="next">Troubleshooting</a>, Previous: <a href="Multi_002dparameter-fitting.html#Multi_002dparameter-fitting" accesskey="p" rel="previous">Multi-parameter fitting</a>, Up: <a href="Least_002dSquares-Fitting.html#Least_002dSquares-Fitting" accesskey="u" rel="up">Least-Squares Fitting</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Robust-linear-regression-1"></a>
<h3 class="section">37.5 Robust linear regression</h3>
<a name="index-robust-regression"></a>
<p>Ordinary least squares (OLS) models are often heavily influenced by the presence of outliers.
Outliers are data points which do not follow the general trend of the other observations,
although there is strictly no precise definition of an outlier. Robust linear regression
refers to regression algorithms which are robust to outliers. The most common type of
robust regression is M-estimation. The general M-estimator minimizes the objective function
where <em>e_i = y_i - Y(c, x_i)</em> is the residual of the ith data point, and
<em>\rho(e_i)</em> is a function which should have the following properties:
</p><ul class="no-bullet">
<li><!-- /@w --> <em>\rho(e) \ge 0</em>
</li><li><!-- /@w --> <em>\rho(0) = 0</em>
</li><li><!-- /@w --> <em>\rho(-e) = \rho(e)</em>
</li><li><!-- /@w --> <em>\rho(e_1) > \rho(e_2)</em> for <em>|e_1| > |e_2|</em>
</li></ul>
<p>The special case of ordinary least squares is given by <em>\rho(e_i) = e_i^2</em>.
Letting <em>\psi = \rho'</em> be the derivative of <em>\rho</em>, differentiating
the objective function with respect to the coefficients <em>c</em>
and setting the partial derivatives to zero produces the system of equations
where <em>X_i</em> is a vector containing row <em>i</em> of the design matrix <em>X</em>.
Next, we define a weight function <em>w(e) = \psi(e)/e</em>, and let
<em>w_i = w(e_i)</em>:
This system of equations is equivalent to solving a weighted ordinary least squares
problem, minimizing <em>\chi^2 = \sum_i w_i e_i^2</em>. The weights however, depend
on the residuals <em>e_i</em>, which depend on the coefficients <em>c</em>, which depend
on the weights. Therefore, an iterative solution is used, called Iteratively Reweighted
Least Squares (IRLS).
</p><ol>
<li> Compute initial estimates of the coefficients <em>c^{(0)}</em> using ordinary least squares
</li><li> For iteration <em>k</em>, form the residuals <em>e_i^{(k)} = (y_i - X_i c^{(k-1)})/(t \sigma^{(k)} \sqrt{1 - h_i})</em>,
where <em>t</em> is a tuning constant depending on the choice of <em>\psi</em>, and <em>h_i</em> are the
statistical leverages (diagonal elements of the matrix <em>X (X^T X)^{-1} X^T</em>). Including <em>t</em>
and <em>h_i</em> in the residual calculation has been shown to improve the convergence of the method.
The residual standard deviation is approximated as <em>\sigma^{(k)} = MAD / 0.6745</em>, where MAD is the
Median-Absolute-Deviation of the <em>n-p</em> largest residuals from the previous iteration.
</li><li> Compute new weights <em>w_i^{(k)} = \psi(e_i^{(k)})/e_i^{(k)}</em>.
</li><li> Compute new coefficients <em>c^{(k)}</em> by solving the weighted least squares problem with
weights <em>w_i^{(k)}</em>.
</li><li> Steps 2 through 4 are iterated until the coefficients converge or until some maximum iteration
limit is reached. Coefficients are tested for convergence using the critera:
for all <em>0 \le i < p</em> where <em>\epsilon</em> is a small tolerance factor.
</li></ol>
<p>The key to this method lies in selecting the function <em>\psi(e_i)</em> to assign
smaller weights to large residuals, and larger weights to smaller residuals. As
the iteration proceeds, outliers are assigned smaller and smaller weights, eventually
having very little or no effect on the fitted model.
</p>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005falloc"></a>Function: <em>gsl_multifit_robust_workspace *</em> <strong>gsl_multifit_robust_alloc</strong> <em>(const gsl_multifit_robust_type * <var>T</var>, const size_t <var>n</var>, const size_t <var>p</var>)</em></dt>
<dd><a name="index-gsl_005fmultifit_005frobust_005fworkspace"></a>
<p>This function allocates a workspace for fitting a model to <var>n</var>
observations using <var>p</var> parameters. The type <var>T</var> specifies the
function <em>\psi</em> and can be selected from the following choices.
</p><dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fdefault"></a>Robust type: <strong>gsl_multifit_robust_default</strong></dt>
<dd><p>This specifies the <code>gsl_multifit_robust_bisquare</code> type (see below) and is a good
general purpose choice for robust regression.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fbisquare"></a>Robust type: <strong>gsl_multifit_robust_bisquare</strong></dt>
<dd><p>This is Tukey’s biweight (bisquare) function and is a good general purpose choice for
robust regression. The weight function is given by
and the default tuning constant is <em>t = 4.685</em>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fcauchy"></a>Robust type: <strong>gsl_multifit_robust_cauchy</strong></dt>
<dd><p>This is Cauchy’s function, also known as the Lorentzian function.
This function does not guarantee a unique solution,
meaning different choices of the coefficient vector <var>c</var>
could minimize the objective function. Therefore this option should
be used with care. The weight function is given by
and the default tuning constant is <em>t = 2.385</em>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005ffair"></a>Robust type: <strong>gsl_multifit_robust_fair</strong></dt>
<dd><p>This is the fair <em>\rho</em> function, which guarantees a unique solution and
has continuous derivatives to three orders. The weight function is given by
and the default tuning constant is <em>t = 1.400</em>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fhuber"></a>Robust type: <strong>gsl_multifit_robust_huber</strong></dt>
<dd><p>This specifies Huber’s <em>\rho</em> function, which is a parabola in the vicinity of zero and
increases linearly for a given threshold <em>|e| > t</em>. This function is also considered
an excellent general purpose robust estimator, however, occasional difficulties can
be encountered due to the discontinuous first derivative of the <em>\psi</em> function.
The weight function is given by
and the default tuning constant is <em>t = 1.345</em>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fols"></a>Robust type: <strong>gsl_multifit_robust_ols</strong></dt>
<dd><p>This specifies the ordinary least squares solution, which can be useful for quickly
checking the difference between the various robust and OLS solutions. The weight function is given by
and the default tuning constant is <em>t = 1</em>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fwelsch"></a>Robust type: <strong>gsl_multifit_robust_welsch</strong></dt>
<dd><p>This specifies the Welsch function which can perform well in cases where the residuals have an
exponential distribution. The weight function is given by
and the default tuning constant is <em>t = 2.985</em>.
</p></dd></dl>
</dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005ffree"></a>Function: <em>void</em> <strong>gsl_multifit_robust_free</strong> <em>(gsl_multifit_robust_workspace * <var>w</var>)</em></dt>
<dd><p>This function frees the memory associated with the workspace <var>w</var>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fname"></a>Function: <em>const char *</em> <strong>gsl_multifit_robust_name</strong> <em>(const gsl_multifit_robust_workspace * <var>w</var>)</em></dt>
<dd><p>This function returns the name of the robust type <var>T</var> specified to <code>gsl_multifit_robust_alloc</code>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005ftune"></a>Function: <em>int</em> <strong>gsl_multifit_robust_tune</strong> <em>(const double <var>tune</var>, gsl_multifit_robust_workspace * <var>w</var>)</em></dt>
<dd><p>This function sets the tuning constant <em>t</em> used to adjust the residuals at each iteration to <var>tune</var>.
Decreasing the tuning constant increases the downweight assigned to large residuals, while increasing
the tuning constant decreases the downweight assigned to large residuals.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust"></a>Function: <em>int</em> <strong>gsl_multifit_robust</strong> <em>(const gsl_matrix * <var>X</var>, const gsl_vector * <var>y</var>, gsl_vector * <var>c</var>, gsl_matrix * <var>cov</var>, gsl_multifit_robust_workspace * <var>w</var>)</em></dt>
<dd><p>This function computes the best-fit parameters <var>c</var> of the model
<em>y = X c</em> for the observations <var>y</var> and the matrix of
predictor variables <var>X</var>, attemping to reduce the influence
of outliers using the algorithm outlined above.
The <em>p</em>-by-<em>p</em> variance-covariance matrix of the model parameters
<var>cov</var> is estimated as <em>\sigma^2 (X^T X)^{-1}</em>, where <em>\sigma</em> is
an approximation of the residual standard deviation using the theory of robust
regression. Special care must be taken when estimating <em>\sigma</em> and
other statistics such as <em>R^2</em>, and so these
are computed internally and are available by calling the function
<code>gsl_multifit_robust_statistics</code>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fest"></a>Function: <em>int</em> <strong>gsl_multifit_robust_est</strong> <em>(const gsl_vector * <var>x</var>, const gsl_vector * <var>c</var>, const gsl_matrix * <var>cov</var>, double * <var>y</var>, double * <var>y_err</var>)</em></dt>
<dd><p>This function uses the best-fit robust regression coefficients
<var>c</var> and their covariance matrix
<var>cov</var> to compute the fitted function value
<var>y</var> and its standard deviation <var>y_err</var> for the model <em>y = x.c</em>
at the point <var>x</var>.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmultifit_005frobust_005fstatistics"></a>Function: <em>gsl_multifit_robust_stats</em> <strong>gsl_multifit_robust_statistics</strong> <em>(const gsl_multifit_robust_workspace * <var>w</var>)</em></dt>
<dd><p>This function returns a structure containing relevant statistics from a robust regression. The function
<code>gsl_multifit_robust</code> must be called first to perform the regression and calculate these statistics.
The returned <code>gsl_multifit_robust_stats</code> structure contains the following fields.
</p><ul class="no-bullet">
<li><!-- /@w --> double <code>sigma_ols</code> This contains the standard deviation of the residuals as computed from ordinary least squares (OLS).
</li><li><!-- /@w --> double <code>sigma_mad</code> This contains an estimate of the standard deviation of the final residuals using the Median-Absolute-Deviation statistic
</li><li><!-- /@w --> double <code>sigma_rob</code> This contains an estimate of the standard deviation of the final residuals from the theory of robust regression (see Street et al, 1988).
</li><li><!-- /@w --> double <code>sigma</code> This contains an estimate of the standard deviation of the final residuals by attemping to reconcile <code>sigma_rob</code> and <code>sigma_ols</code>
in a reasonable way.
</li><li><!-- /@w --> double <code>Rsq</code> This contains the <em>R^2</em> coefficient of determination statistic using the estimate <code>sigma</code>.
</li><li><!-- /@w --> double <code>adj_Rsq</code> This contains the adjusted <em>R^2</em> coefficient of determination statistic using the estimate <code>sigma</code>.
</li><li><!-- /@w --> double <code>rmse</code> This contains the root mean squared error of the final residuals
</li><li><!-- /@w --> double <code>sse</code> This contains the residual sum of squares taking into account the robust covariance matrix.
</li><li><!-- /@w --> size_t <code>dof</code> This contains the number of degrees of freedom <em>n - p</em>
</li><li><!-- /@w --> size_t <code>numit</code> Upon successful convergence, this contains the number of iterations performed
</li><li><!-- /@w --> gsl_vector * <code>weights</code> This contains the final weight vector of length <var>n</var>
</li><li><!-- /@w --> gsl_vector * <code>r</code> This contains the final residual vector of length <var>n</var>, <em>r = y - X c</em>
</li></ul>
</dd></dl>
<hr>
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Next: <a href="Troubleshooting.html#Troubleshooting" accesskey="n" rel="next">Troubleshooting</a>, Previous: <a href="Multi_002dparameter-fitting.html#Multi_002dparameter-fitting" accesskey="p" rel="previous">Multi-parameter fitting</a>, Up: <a href="Least_002dSquares-Fitting.html#Least_002dSquares-Fitting" accesskey="u" rel="up">Least-Squares Fitting</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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