polynomial
Polynomial surrogate model
Specification
Alias: None
Arguments: None
Child Keywords:
Required/Optional 
Description of Group 
Dakota Keyword 
Dakota Keyword Description 

Required (Choose One) 
Polynomial Order 
Polynomial order 

Use a linear polynomial or trend function 

Use a quadratic polynomial or trend function 

Use a cubic polynomial 

Optional 
Exports surrogate model in userspecified format(s) 

Optional 
Import surrogate model from archive file 
Description
Linear, quadratic, and cubic polynomial surrogate models are available in Dakota. The utility of the simple polynomial models stems from two sources:
over a small portion of the parameter space, a loworder polynomial model is often an accurate approximation to the true data trends
the leastsquares procedure provides a surface fit that smooths out noise in the data.
Local surrogatebased optimization methods (
surrogate_based_local
) are often successful when using
polynomial models, particularly quadratic models. However, a
polynomial surface fit may not be the best choice for modeling data
trends globally over the entire parameter space, unless it is known a
priori that the true data trends are close to linear, quadratic, or
cubic. See [MM95] for more information on polynomial models.
Known Issue: When using discrete variables, there have been sometimes significant differences in surrogate behavior observed across computing platforms in some cases. The cause has not yet been fully diagnosed and is currently under investigation. In addition, guidance on appropriate construction and use of surrogates with discrete variables is under development. In the meantime, users should therefore be aware that there is a risk of inaccurate results when using surrogates with discrete variables.
Theory
The form of the linear polynomial model is
the form of the quadratic polynomial model is:
and the form of the cubic polynomial model is:
In all of the polynomial models, \(\hat{f}(\mathbf{x})\) is the response of the polynomial model; the \(x_{i},x_{j},x_{k}\) terms are the components of the \(n\) dimensional design parameter values; the \(c_{0}\) , \(c_{i}\) , \(c_{ij}\) , \(c_{ijk}\) terms are the polynomial coefficients, and \(n\) is the number of design parameters. The number of coefficients, \(n_{c}\) , depends on the order of polynomial model and the number of design parameters. For the linear polynomial:
for the quadratic polynomial:
and for the cubic polynomial:
There must be at least \(n_{c}\) data samples in order to form a fully determined linear system and solve for the polynomial coefficients. In Dakota, a leastsquares approach involving a singular value decomposition numerical method is applied to solve the linear system.