Correlator {hmer}

Correlation Structure

Description

Creates a correlation structure, with the necessary specifications.

The correlator has three main elements: the type of correlator, the associated hyperparameters, and the nugget term. The nugget term is broadly separate from the other two parameters, being type-independent.

Constructor

Correlator$new(corr, hp, nug)

Arguments

corr The type of correlation function. This is provided as a string which corresponds exactly with a function - the function should take three arguments x, xp, hp. This gives a correlation function u(x, xp) defined by hyperparameters hp. For a simple example, see exp_sq.

hp The associated hyperparameters needed to define the correlation structure, as a named list. In the case of exp_sq, this is a list of one element, list(theta).

nug The size of the nugget term. In situations where not all variables are active, the main part of u(x) operates only on the active parts, xA. The presence of the nugget term accounts for the fact that points at the same position in the active space need not be at the same position in the full space.

By default, Correlator$new() initialises with corr = exp_sq, hp = list(theta = 0.1), and nug = 0.

Accessor Methods

get_corr(x, xp = NULL, actives = TRUE) Returns the correlation between two points. If xp is NULL, then this is correlation between a set of points and themselves (i.e. 1 on the diagonal). All variables are assumed to be active unless otherwise stated in actives.

get_hyper_p() Returns the list of hyperparameters.

print() Produces a summary of the correlation structure specification.

Object Methods

set_hyper_p(hp, nugget) Modifies the hyperparameter and/or nugget terms. Returns a new Correlator object.

Options for Correlations

The default choice (and that supported by other functions in this package, particularly emulator_from_data) for the correlation structure is exponential-squared, due to the useful properties it possesses. However, one can manually instantiate a Correlator with a different underlying structure. Built-in alternatives are as follows, as well as whether a form exists for its derivative:

matern

the Matérn function (derivative exists)

orn_uhl

the Ornstein-Uhlenbeck function (no derivative)

rat_quad

the rational quadratic function (derivative exists)

One more function, gamma_exp, is available but not directly supported by emulator_from_data, for example, due to its very limited suitability to emulating model outputs. However, this can be used as a test case for writing one's own correlation functions and using them with emulator_from_data.

A user-defined correlation function can be provided to the Correlator: the requirements are that the function accept data.matrix objects as its first and second arguments, and accept a named list of hyperparameters as its third argument, and return a matrix of correlations between rows of the data.matrices. If a derivative also exists, it should take the same name as the correlation function with "_d" appended to it, and the directions to differentiate with respect to should come after the hyperparameter argument. For example, the rational quadratic functions have the form

rat_quad(x1, x2, hp = list(alpha, theta))

rat_quad_d(x1, x2, hp = list(alpha, theta), dx1, dx2)

If defining a custom correlation function, care should be taken with hyperparameter estimation - see emulator_from_data examples for details.

Examples

test_corr <- Correlator$new(nug = 0.1)
test_corr
point1 <- data.frame(a = 0.1, b = 0.2, c = 0.3)
point2 <- data.frame(a = 0.15, b = 0.18, c = 0.295)
test_corr$get_corr(point1) #> 1
test_corr$get_corr(point1, point2) #> 0.6717557
test_corr$get_corr(point1, point2, actives = c(TRUE, TRUE, FALSE)) #> 0.6734372

new_corr <- test_corr$set_hyper_p(list(theta = 0.5), nug = 0.01)
new_corr$get_corr(point1, point2) #> 0.9784845
new_corr$get_corr(point1, point2, actives = c(TRUE, TRUE, FALSE)) #> 0.9785824

mat_corr <- Correlator$new('matern', list(nu = 1.5, theta = 0.5))
mat_corr$get_corr(data.frame(a = c(1, 0.9), b = c(4, 4.2)))