# GeneralLinearModelAlgorithm¶

class GeneralLinearModelAlgorithm(*args)

Algorithm for the evaluation of general linear models.

Available constructors:

GeneralLinearModelAlgorithm(inputSample, outputSample, covarianceModel, basis, normalize=True, keepCovariance=True)

GeneralLinearModelAlgorithm(inputSample, outputSample, covarianceModel, basisCollection, normalize=True, keepCovariance=True)

Parameters: inputSample, outputSample : Sample or 2d-array The samples and . inputTransformation : Function Function used to normalize the input sample. If used, the meta model is built on the transformed data. basis : Basis Functional basis to estimate the trend: . If , the same basis is used for each marginal output. basisCollection : collection of Basis Collection of functional basis: one basis for each marginal output. An empty collection means that no trend is estimated. covarianceModel : CovarianceModel Covariance model of the Gaussian process. See notes for the details. normalize : bool, optional Indicates whether the input sample has to be normalized. OpenTURNS uses the transformation fixed by the User in inputTransformation or the empirical mean and variance of the input sample. Default is set in resource map key GeneralLinearModelAlgorithm-NormalizeData keepCovariance : bool, optional Indicates whether the covariance matrix has to be stored in the result structure GeneralLinearModelResult. Default is set in resource map key GeneralLinearModelAlgorithm-KeepCovariance

Notes

We suppose we have a sample where for all , with a given function.

The objective is to build a metamodel , using a general linear model: the sample is considered as the restriction of a Gaussian process on . The Gaussian process is defined by:

where:

with and the trend functions.

is a Gaussian process of dimension with zero mean and covariance function (see CovarianceModel for the notations).

We note:

The GeneralLinearModelAlgorithm class estimates the coefficients where is the vector of parameters of the covariance model (a subset of ) that has been declared as active (by default, the full vectors and ).

The estimation is done by maximizing the reduced log-likelihood of the model, see its expression below.

If a normalizing transformation has been used, the meta model is built on the inputs .

Estimation of the parameters

We note:

where .

The model likelihood writes:

If is the Cholesky factor of , ie the lower triangular matrix with positive diagonal such that , then:

(1)

The maximization of (1) leads to the following optimality condition for :

This expression of as a function of is taken as a general relation between and and is substituted into (1), leading to a reduced log-likelihood function depending solely on .

In the particular case where and is a part of , then a further reduction is possible. In this case, if is the vector in which has been substituted by 1, then:

showing that is a function of only, and the optimality condition for reads:

which leads to a further reduction of the log-likelihood function where both and are replaced by their expression in terms of .

The default optimizer is TNC and can be changed thanks to the setOptimizationAlgorithm method. User could also change the default optimization solver by setting the GeneralLinearModelAlgorithm-DefaultOptimizationAlgorithm resource map key to one of the NLopt solver names.

It is also possible to proceed as follows:

• ask for the reduced log-likelihood function of the GeneralLinearModelAlgorithm thanks to the getObjectiveFunction() method
• optimize it with respect to the parameters and using any optimization algorithms (that can take into account some additional constraints if needed)
• set the optimal parameter value into the covariance model used in the GeneralLinearModelAlgorithm
• tell the algorithm not to optimize the parameter using setOptimizeParameters
The behaviour of the reduction is controlled by the following keys in ResourceMap:
• ResourceMap.SetAsBool(‘GeneralLinearModelAlgorithm-UseAnalyticalAmplitudeEstimate’, true) to use the reduction associated to . It has no effect if or if and is not part of
• ResourceMap.SetAsBool(‘GeneralLinearModelAlgorithm-UnbiasedVariance’, true) allows to use the unbiased estimate of where is replaced by in the optimality condition for .

With huge samples, the hierarchical matrix implementation could be used if OpenTURNS had been compiled with hmat-oss support.

This implementation, which is based on a compressed representation of an approximated covariance matrix (and its Cholesky factor), has a better complexity both in terms of memory requirements and floating point operations. To use it, the GeneralLinearModelAlgorithm-LinearAlgebra resource map key should be instancied to HMAT. Default value of the key is LAPACK.

A known centered gaussian observation noise can be taken into account with setNoise():

Examples

Create the model and the samples:

>>> import openturns as ot
>>> f = ot.SymbolicFunction(['x'], ['x * sin(x)'])
>>> inputSample = ot.Sample([[1.0], [3.0], [5.0], [6.0], [7.0], [8.0]])
>>> outputSample = f(inputSample)


Create the algorithm:

>>> basis = ot.ConstantBasisFactory().build()
>>> covarianceModel = ot.SquaredExponential(1)
>>> algo = ot.GeneralLinearModelAlgorithm(inputSample, outputSample, covarianceModel, basis)
>>> algo.run()


Get the resulting meta model:

>>> result = algo.getResult()
>>> metamodel = result.getMetaModel()


Methods

 getClassName() Accessor to the object’s name. getDistribution() Accessor to the joint probability density function of the physical input vector. getId() Accessor to the object’s id. getInputSample() Accessor to the input sample. getInputTransformation() Get the function normalizing the input. getName() Accessor to the object’s name. getNoise() Observation noise variance accessor. getObjectiveFunction() Accessor to the log-likelihood function that writes as argument of the covariance’s model parameters. getOptimizationAlgorithm() Accessor to solver used to optimize the covariance model parameters. getOptimizationBounds() Optimization bounds accessor. getOptimizeParameters() Accessor to the covariance model parameters optimization flag. getOutputSample() Accessor to the output sample. getResult() Get the results of the metamodel computation. getShadowedId() Accessor to the object’s shadowed id. getVisibility() Accessor to the object’s visibility state. hasName() Test if the object is named. hasVisibleName() Test if the object has a distinguishable name. run() Compute the response surface. setDistribution(distribution) Accessor to the joint probability density function of the physical input vector. setInputTransformation(inputTransformation) Set the function normalizing the input. setName(name) Accessor to the object’s name. setNoise(noise) Observation noise variance accessor. setOptimizationAlgorithm(solver) Accessor to the solver used to optimize the covariance model parameters. setOptimizationBounds(optimizationBounds) Optimization bounds accessor. setOptimizeParameters(optimizeParameters) Accessor to the covariance model parameters optimization flag. setShadowedId(id) Accessor to the object’s shadowed id. setVisibility(visible) Accessor to the object’s visibility state.
__init__(*args)

Initialize self. See help(type(self)) for accurate signature.

getClassName()

Accessor to the object’s name.

Returns: class_name : str The object class name (object.__class__.__name__).
getDistribution()

Accessor to the joint probability density function of the physical input vector.

Returns: distribution : Distribution Joint probability density function of the physical input vector.
getId()

Accessor to the object’s id.

Returns: id : int Internal unique identifier.
getInputSample()

Accessor to the input sample.

Returns: inputSample : Sample The input sample .
getInputTransformation()

Get the function normalizing the input.

Returns: transformation : Function Function T that normalizes the input.
getName()

Accessor to the object’s name.

Returns: name : str The name of the object.
getNoise()

Observation noise variance accessor.

Parameters: noise : sequence of positive float The noise variance of each output value.
getObjectiveFunction()

Accessor to the log-likelihood function that writes as argument of the covariance’s model parameters.

Returns: logLikelihood : Function The log-likelihood function degined in (1) as a function of .

Notes

The log-likelihood function may be useful for some postprocessing: maximization using external optimizers for example.

Examples

Create the model and the samples:

>>> import openturns as ot
>>> f = ot.SymbolicFunction(['x0'], ['x0 * sin(x0)'])
>>> inputSample = ot.Sample([[1.0], [3.0], [5.0], [6.0], [7.0], [8.0]])
>>> outputSample = f(inputSample)


Create the algorithm:

>>> basis = ot.ConstantBasisFactory().build()
>>> covarianceModel = ot.SquaredExponential(1)
>>> algo = ot.GeneralLinearModelAlgorithm(inputSample, outputSample, covarianceModel, basis)
>>> algo.run()


Get the log-likelihood function:

>>> likelihoodFunction = algo.getObjectiveFunction()

getOptimizationAlgorithm()

Accessor to solver used to optimize the covariance model parameters.

Returns: algorithm : OptimizationAlgorithm Solver used to optimize the covariance model parameters. Default optimizer is TNC
getOptimizationBounds()

Optimization bounds accessor.

Returns: bounds : Interval Bounds for covariance model parameter optimization.
getOptimizeParameters()

Accessor to the covariance model parameters optimization flag.

Returns: optimizeParameters : bool Whether to optimize the covariance model parameters.
getOutputSample()

Accessor to the output sample.

Returns: outputSample : Sample The output sample .
getResult()

Get the results of the metamodel computation.

Returns: result : GeneralLinearModelResult Structure containing all the results obtained after computation and created by the method run().
getShadowedId()

Accessor to the object’s shadowed id.

Returns: id : int Internal unique identifier.
getVisibility()

Accessor to the object’s visibility state.

Returns: visible : bool Visibility flag.
hasName()

Test if the object is named.

Returns: hasName : bool True if the name is not empty.
hasVisibleName()

Test if the object has a distinguishable name.

Returns: hasVisibleName : bool True if the name is not empty and not the default one.
run()

Compute the response surface.

Notes

It computes the response surface and creates a GeneralLinearModelResult structure containing all the results.

setDistribution(distribution)

Accessor to the joint probability density function of the physical input vector.

Parameters: distribution : Distribution Joint probability density function of the physical input vector.
setInputTransformation(inputTransformation)

Set the function normalizing the input.

Parameters: transformation : Function Function that normalizes the input. The input dimension should be the same as input’s sample dimension, output dimension should be output sample’s dimension
setName(name)

Accessor to the object’s name.

Parameters: name : str The name of the object.
setNoise(noise)

Observation noise variance accessor.

Parameters: noise : sequence of positive float The noise variance of each output value.
setOptimizationAlgorithm(solver)

Accessor to the solver used to optimize the covariance model parameters.

Parameters: algorithm : OptimizationAlgorithm Solver used to optimize the covariance model parameters.
setOptimizationBounds(optimizationBounds)

Optimization bounds accessor.

Parameters: bounds : Interval Bounds for covariance model parameter optimization.

Notes

Parameters involved by this method are:

• Scale parameters,
• Amplitude parameters if output dimension is greater than one or analytical sigma disabled,

Lower & upper bounds are defined in resource map. Default lower upper bounds value for all parameters is and defined thanks to the GeneralLinearModelAlgorithm-DefaultOptimizationLowerBound resource map key.

For scale parameters, default upper bounds are set as times the difference between the max and min values of X for each coordinate, X being the (transformed) input sample. The value is defined in resource map (GeneralLinearModelAlgorithm-DefaultOptimizationScaleFactor).

Finally for other parameters (amplitude,…), default upper bound is set to (corresponding resource map key is GeneralLinearModelAlgorithm-DefaultOptimizationUpperBound)

setOptimizeParameters(optimizeParameters)

Accessor to the covariance model parameters optimization flag.

Parameters: optimizeParameters : bool Whether to optimize the covariance model parameters.
setShadowedId(id)

Accessor to the object’s shadowed id.

Parameters: id : int Internal unique identifier.
setVisibility(visible)

Accessor to the object’s visibility state.

Parameters: visible : bool Visibility flag.