subselect (version 0.15.5)

wald.coef: Wald statistic for variable selection in generalized linear models

Description

Computes the value of Wald's statistic, testing the significance of the excluded variables, in the context of variable subset selection in generalized linear models

Usage

wald.coef(mat, H, indices,
tolval=10*.Machine$double.eps, tolsym=1000*.Machine$double.eps)

Value

The value of the Wald statistic.

Arguments

mat

An estimate (FI) of Fisher's information matrix for the full model variable-coefficient estimates

H

A matrix product of the form FI %*% b %*% t(b) %*% FI where b is a vector of variable-coefficient estimates

indices

a numerical vector, matrix or 3-d array of integers giving the indices of the variables in the subset. If a matrix is specified, each row is taken to represent a different k-variable subset. If a 3-d array is given, it is assumed that the third dimension corresponds to different cardinalities.

tolval

the tolerance level to be used in checks for ill-conditioning and positive-definiteness of the Fisher Information and the auxiliar (H) matrices. Values smaller than tolval are considered equivalent to zero.

tolsym

the tolerance level for symmetry of the Fisher Information and the auxiliar (H) matrices. If corresponding matrix entries differ by more than this value, the input matrices will be considered asymmetric and execution will be aborted. If corresponding entries are different, but by less than this value, the input matrix will be replaced by its symmetric part, i.e., input matrix A becomes (A+t(A))/2.

Details

Variable selection in the context of generalized linear models is typically based on the minimization of statistics that test the significance of excluded variables. In particular, the likelihood ratio, Wald's, Rao's and some adaptations of such statistics, are often proposed as comparison criteria for variable subsets of the same dimensionality. All these statistics are assympotically equivalent and can be converted into information criteria, like the AIC, that are also able to compare subsets of different dimensionalities (see references [1] and [2] for further details).

Among these criteria, Wald's statistic has some computational advantages because it can always be derived from the same (concerning the full model) maximum likelihood and Fisher information estimates. In particular, if \(W_{allv}\) is the value of the Wald statistic testing the significance of the full covariate vector, b and FI are coefficient and Fisher information estimates and H is an auxiliary rank-one matrix given by H = FI %*% b %*% t(b) %*% FI, it follows that the value of Wald's statistic for the excluded variables (\(W_{excv}\)) in a given subset is given by \(W_{excv} = W_{allv} - tr (FI_{indices}^{-1} H_{indices}) ,\) where \(FI_{indices}\) and \(H_{indices}\) are the portions of the FI and H matrices associated with the selected variables.

The FI and H matrices can be retrieved (from a glm object) by the glmHmat function and may be used as input to the search functions anneal, genetic, improve and eleaps. The Wald function computes the value of Wald statistc from these matrices for a subset specified by indices

The fact that indices can be a matrix or 3-d array allows for the computation of the Wald statistic values of subsets produced by the search functions anneal, genetic, improve and eleaps (whose output option $subsets are matrices or 3-d arrays), using a different criterion (see the example below).

References

[1] Lawless, J. and Singhal, K. (1978). Efficient Screening of Nonnormal Regression Models, Biometrics, Vol. 34, 318-327.

[2] Lawless, J. and Singhal, K. (1987). ISMOD: An All-Subsets Regression Program for Generalized Models I. Statistical and Computational Background, Computer Methods and Programs in Biomedicine, Vol. 24, 117-124.

Examples

## ---------------------------------------------------------------


##  An example of variable selection in the context of binary response
##  regression models. The logarithms and original physical measurements
##  of the "Leptograpsus variegatus crabs" considered in the MASS crabs 
##  data set are used to fit a logistic model that takes the sex of each crab
##  as the response variable.

library(MASS)
data(crabs)
lFL <- log(crabs$FL)
lRW <- log(crabs$RW)
lCL <- log(crabs$CL)
lCW <- log(crabs$CW)
logrfit <- glm(sex ~ FL + RW + CL + CW  + lFL + lRW + lCL + lCW,
crabs,family=binomial) 
## Warning message:
## fitted probabilities numerically 0 or 1 occurred in: glm.fit(x = X, y = Y, 
## weights = weights, start = start, etastart = etastart, 

lHmat <- glmHmat(logrfit) 
wald.coef(lHmat$mat,lHmat$H,c(1,6,7),tolsym=1E-06)
## [1] 2.286739
## Warning message:

## The covariance/total matrix supplied was slightly asymmetric: 
## symmetric entries differed by up to 6.57252030578093e-14.
## (less than the 'tolsym' parameter).
## It has been replaced by its symmetric part.
## in: validmat(mat, p, tolval, tolsym)


## ---------------------------------------------------------------

## 2) An example computing the value of the Wald statistic in a logistic 
##  model for five subsets produced when a probit model was originally 
##  considered

library(MASS)
data(crabs)
lFL <- log(crabs$FL)
lRW <- log(crabs$RW)
lCL <- log(crabs$CL)
lCW <- log(crabs$CW)
probfit <- glm(sex ~ FL + RW + CL + CW  + lFL + lRW + lCL + lCW,
crabs,family=binomial(link=probit)) 
## Warning message:
## fitted probabilities numerically 0 or 1 occurred in: glm.fit(x = X, y = Y, 
## weights = weights, start = start, etastart = etastart) 

pHmat <- glmHmat(probfit) 
probresults <-eleaps(pHmat$mat,kmin=3,kmax=3,nsol=5,criterion="Wald",H=pHmat$H,
r=1,tolsym=1E-10)
## Warning message:

## The covariance/total matrix supplied was slightly asymmetric: 
## symmetric entries differed by up to 3.14059889205964e-12.
## (less than the 'tolsym' parameter).
## It has been replaced by its symmetric part.
## in: validmat(mat, p, tolval, tolsym) 

logrfit <- glm(sex ~ FL + RW + CL + CW  + lFL + lRW + lCL + lCW,
crabs,family=binomial) 
## Warning message:
## fitted probabilities numerically 0 or 1 occurred in: glm.fit(x = X, y = Y, 
## weights = weights, start = start, etastart = etastart)

lHmat <- glmHmat(logrfit) 
wald.coef(lHmat$mat,H=lHmat$H,probresults$subsets,tolsym=1e-06)
##             Card.3
## Solution 1 2.286739
## Solution 2 2.595165
## Solution 3 2.585149
## Solution 4 2.669059
## Solution 5 2.690954
## Warning message:

## The covariance/total matrix supplied was slightly asymmetric: 
## symmetric entries differed by up to 6.57252030578093e-14.
## (less than the 'tolsym' parameter).
## It has been replaced by its symmetric part.
## in: validmat(mat, p, tolval, tolsym)