.. _model-surrogate-global-gaussian_process-surfpack-trend:

"""""
trend
"""""


Choose a trend function for a Gaussian process surrogate


.. toctree::
   :hidden:
   :maxdepth: 1

   model-surrogate-global-gaussian_process-surfpack-trend-constant
   model-surrogate-global-gaussian_process-surfpack-trend-linear
   model-surrogate-global-gaussian_process-surfpack-trend-reduced_quadratic
   model-surrogate-global-gaussian_process-surfpack-trend-quadratic


**Specification**

- *Alias:* None

- *Arguments:* None

- *Default:* reduced_quadratic


**Child Keywords:**

+-------------------------+--------------------+-----------------------+-----------------------------------------------+
| Required/Optional       | Description of     | Dakota Keyword        | Dakota Keyword Description                    |
|                         | Group              |                       |                                               |
+=========================+====================+=======================+===============================================+
| Required (Choose One)   | Trend Order        | `constant`__          | Constant trend function                       |
|                         |                    +-----------------------+-----------------------------------------------+
|                         |                    | `linear`__            | Use a linear polynomial or trend function     |
|                         |                    +-----------------------+-----------------------------------------------+
|                         |                    | `reduced_quadratic`__ | Quadratic polynomials - main effects only     |
|                         |                    +-----------------------+-----------------------------------------------+
|                         |                    | `quadratic`__         | Use a quadratic polynomial or trend function  |
+-------------------------+--------------------+-----------------------+-----------------------------------------------+

.. __: model-surrogate-global-gaussian_process-surfpack-trend-constant.html
__ model-surrogate-global-gaussian_process-surfpack-trend-linear.html
__ model-surrogate-global-gaussian_process-surfpack-trend-reduced_quadratic.html
__ model-surrogate-global-gaussian_process-surfpack-trend-quadratic.html



**Description**


The only trend functions that are currently supported are polynomials.

The trend function is selected using the ``trend``
keyword, with options ``constant``, ``linear``, or ``reduced_quadratic``.
The ``reduced_quadratic`` trend function includes the main
effects, but not mixed/interaction terms.
The Surfpack GP (See :dakkw:`model-surrogate-global-gaussian_process-surfpack`)
has the additional option of (a full)
``quadratic``.


