Outliers
========

.. figure:: icons/outliers.png

Simple outlier detection by comparing distances between instances.

Signals
-------

**Inputs**:

-  **Data**

   A data set

-  **Distances**

   A distance matrix

**Outputs**:

-  **Outliers**

   A data set containing instances scored as outliers

-  **Inliers**

   A data set containing instances not scored as outliers

Description
-----------

The **Outliers** widget applies one of the two methods for outlier
detection. Both methods apply classification to the data set, one with
SVM (multiple kernels) and the other with elliptical envelope.
*One-class SVM with non-linear kernels (RBF)* performs well with
non-Gaussian distributions, while *Covariance estimator* works only for
data with Gaussian distribution.

.. figure:: images/Outliers-stamped.png

1. Information on the input data, number of inliers and outliers based
   on the selected model.
2. Select the *Outlier detection method*:

   -  **One class SVM with non-linear kernel (RBF)**: classifies data as
      similar or different from the core class

      -  **Nu** is a parameter for the upper bound on the fraction of
         training errors and a lower bound of the fraction of support
         vectors
      -  **Kernel coefficient** is a gamma parameter, which specifies how
         much influence a single data instance has

   -  **Covariance estimator**: fits ellipsis to central points with
      Mahalanobis distance metric

      -  **Contamination** is the proportion of outliers in the data set
      -  **Support fraction** specifies the proportion of points included
         in the estimate

3. Produce a report. 
4. Click *Detect outliers* to output the data.

Example
-------

Below, is a simple example of how to use this widget. We used the *Iris*
data set to detect the outliers. We chose the *one class SVM with
non-linear kernel (RBF)* method, with Nu set at 20% (less training
errors, more support vectors). Then we observed the outliers in the
:doc:`Data Table <../data/datatable>` widget, while we sent the inliers to the :doc:`Scatter
Plot <../visualize/scatterplot>`.

.. figure:: images/Outliers-Example.png