.. _method-polynomial_chaos-refinement_metric:

"""""""""""""""""
refinement_metric
"""""""""""""""""


Metric used for guiding adaptive refinement during UQ.



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

   method-polynomial_chaos-refinement_metric-level_mappings
   method-polynomial_chaos-refinement_metric-covariance


**Specification**

- *Alias:* None

- *Arguments:* None

- *Default:* requested statistics


**Child Keywords:**

+-------------------------+--------------------+--------------------+-----------------------------------------------+
| Required/Optional       | Description of     | Dakota Keyword     | Dakota Keyword Description                    |
|                         | Group              |                    |                                               |
+=========================+====================+====================+===============================================+
| Required (Choose One)   | type of refinement | `level_mappings`__ | Utilize the level mappings metric for guiding |
|                         | metric             |                    | adaptive refinement during UQ.                |
|                         |                    +--------------------+-----------------------------------------------+
|                         |                    | `covariance`__     | Utilize the response covariance metric for    |
|                         |                    |                    | guiding adaptive refinement during UQ.        |
+-------------------------+--------------------+--------------------+-----------------------------------------------+

.. __: method-polynomial_chaos-refinement_metric-level_mappings.html
__ method-polynomial_chaos-refinement_metric-covariance.html



**Description**


The refinement metric for each (greedy) refinement candidate defaults
to the change induced by the candidate across the set of level
mappings (L2 norm in the results for response levels, probability
levels, reliability levels, and generalized reliability levels)
whenever level mappings are specified.  This supports goal orientation
during the refinement process.  When no level mappings are specified,
the default metric is the norm of the change in the response
covariance matrix for the QoI targets.

This specification option allows the user to override the default, in
particular activating the response covariance option despite the
presence of level mappings (see example below).




**Examples**



.. code-block::

    method,
        multifidelity_polynomial_chaos				
	  p_refinement uniform
	    expansion_order_sequence = 0 cross_validation
	    collocation_ratio = 0.75
	    seed = 8674132
	  allocation_control greedy
	    refinement_metric covariance  # override default of level mappings
	    convergence_tolerance = 1.e-6
	  samples_on_emulator = 250000
	  num_response_levels = 41 41 41
	  response_levels = -100:5:100 -100:5:100 -100:5:100