.. _releasenotes-619:

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Version 6.19 (2023/11/15)
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**Highlight: New sampling-based method for Sobol' main effects**

Based on :cite:p:`Li16`, Dakota can now obtain estimates
of first order Sobol’ indices (main effects) from ``sampling``
studies. Previous versions of Dakota could estimate
main and total effects using a "pick and freeze" sampling strategy 
:cite:p:`Sal04`, which typically required a very large number of samples 
(hundreds or thousands per variable) that had to be carefully structured. While
the new method produces only main effects, the requirement on
sample design has been lifted, and typically far fewer samples are
needed for convergence.

*Enabling / Accessing:* 

The :dakkw:`method-sampling-variance_based_decomp` keyword now has 
suboptions. The :dakkw:`vbd_sampling_method pick_and_freeze <method-sampling-variance_based_decomp-vbd_sampling_method-pick_and_freeze>`
option is the default, and causes Dakota to use the method that has long been available
to compute main and total effects. The 
:dakkw:`vbd_sampling_method binned <method-sampling-variance_based_decomp-vbd_sampling_method-binned>`
option causes the new method to be used.

*Documentation:* 

Keyword reference for the :dakkw:`method-sampling-variance_based_decomp-vbd_sampling_method-binned`
VBD method.


**Highlight: Low-discrepancy (quasi-Monte Carlo) sampling**

Two new strategies for choosing low-discrepancy points in sampling
studies are available in this release. These include lattice rules
and digital nets. The well-known Sobol sequence is an example of a
digital net. Just as in Latin hypercube sampling, these strategies
choose points that cover the parameter space more uniformly than ordinary
Monte Carlo, leading to faster convergence of UQ results.

*Enabling / Accessing:* 

In a ``sampling`` study, choose 
:dakkw:`sample_type low_discrepancy <method-sampling-sample_type-low_discrepancy>`.

*Documentation:* 

* :dakkw:`method-sampling-sample_type-low_discrepancy` keyword.
* Discussion of :ref:`low-disrepancy methods <uq:sampling>`.


**Highlight: Model selection in multifidelity sampling methods**

MFMC, ACV, and generalized ACV now support selection of the most
performant subset of model approximations, as determined through
enumeration by the estimator accuracy versus equivalent cost trade-off.

*Enabling / Accessing:* 

In a ``multifidelity_sampling`` or ``approximate_control_variate`` study,
choose :dakkw:`search_model_graphs model_selection <method-approximate_control_variate-search_model_graphs-model_selection>`.

*Documentation:* 

* :dakkw:`method-approximate_control_variate-search_model_graphs-model_selection` keyword.


**Improvements by Category**

*Interfaces, Input/Output*

* Copying of :ref:`dakota.interfacing <interfaces:dakota.interfacing>` objects was improved by adding ``__deepcopy__`` implementations
* New examples:

  * Demonstrating use of Dakota's direct :dakkw:`interface-analysis_drivers-python` interface with a 
    `pre-built tensorflow model <https://github.com/snl-dakota/dakota-examples/tree/master/official/drivers/Python/linked_di/tensorflow>`_.
  * Demonstrating use of ``dakota.interfacing.dprepro`` in a black-box interface.
    (For `Windows <https://github.com/snl-dakota/dakota-examples/tree/master/official/drivers/black-box_simulation_windows>`_
    and `Linux/macOS <https://github.com/snl-dakota/dakota-examples/tree/master/official/drivers/black-box_simulation>`_)
   
*UQ Methods*

* Improved support for Bayesian calibration methods provided by the MIT Uncertainty Quantification Library (MUQ);
  although MUQ is not enabled in our pre-built downloads, it is now buildable within Dakota by a wider variety of toolchains.
* Low discrepancy sampling strategies (see highlight)

*MLMF Sampling*

* Support for selection of the most performant approximation subset, as highlighted above.
* New options for more robust numerical solution of optimal sample allocations, including ``global_local`` and ``competed_local`` solution strategies.  The former is now the default for numerical solutions in MFMC, ACV, and generalized ACV.


*Sensitivity Analysis*

* Binned method for sampling-based variance-based decomposition (see highlight)
* New examples explaining use of
  `correlation coefficients <https://github.com/snl-dakota/dakota-examples/tree/master/official/global_sensitivity/correlations>`_
  for global sensitivity analysis.
 
**Miscellaneous Enhancements and Bugfixes**

- Enh: Documentation of Dakota's regression test system expanded.

- Bug fix: The ``@python_interface`` decorator in the ``dakota.interfacing`` module now propertly
  interprets the dvv list provided by Dakota's direct :dakkw:`interface-analysis_drivers-python` interface.

- Bug fix: RPATH handling on Linux-based platforms improved.

- Enh: :ref:`pyprepro <interfaces:dprepro-and-pyprepro>` gained a new function, ``json_dumps()``, which
  returns all variables (and their values) formatted as a JSON string. See the
  `pyprepro manual <https://github.com/snl-dakota/dakota/blob/devel/scripts/pyprepro/manual.md#json-dumps>`_
  for more information.

**Compatibility**

- Support for building Dakota with C++17 has been greatly expanded and is expected to work for
  GCC, Intel, and Clang compilers. Support for Microsoft Visual Studio in progress.

**Known Issues**

- Plotting in the Dakota GUI on RHEL8 may fail with the error "Unhandled event loop exception. No more handles
  because there is no underlying browser available." There currently is no known workaround or resolution.