.. _model-surrogate-ensemble-truth_model_pointer-approximation_models:

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approximation_models
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Specification of an unordered ensemble of low-fidelity approximations


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



**Specification**

- *Alias:* unordered_model_fidelities 

- *Arguments:* STRINGLIST


**Description**


An unordered or "non-hierarchical" ensemble surrogate manages an unordered set of
low-fidelity model approximations, each of which may include hyper-parameter
resolution controls (in the case of a simulation model) or additional model
recursions.

Any corresponding sequence specifications within methods (e.g.,
``quadrature_order_sequence``, ``sparse_grid_level_sequence``,
``expansion_order_sequence``, etc. within stochastic expansion methods)
should be synchronized with the order in the model listing.

Internal to the ensemble surrogate model, subsets of the model ensemble
may be active for any given evaluation, as dictated by the iterative
algorithm in use.



**Examples**



.. code-block::

    model,
     id_model = 'NONHIERARCH'
     surrogate ensemble
       unordered_model_fidelities = 'LF1' 'LF2'
       truth_model_pointer = 'HF'
    
    model,
     id_model = 'LF1'
     simulation
       interface_pointer = 'LF1_DRIVER'
              solution_level_cost = 1.
    
    model,
     id_model = 'LF2'
     simulation
       interface_pointer = 'LF2_DRIVER'
              solution_level_cost = 2.4
    
    model,
     id_model = 'HF'
     simulation
       interface_pointer = 'HF_DRIVER'
              solution_level_cost = 256.




