postModeOrtho: Bayesian model selection and averaging under block-diagonal...

In mombf: Model Selection with Bayesian Methods and Information Criteria

Bayesian model selection and averaging under block-diagonal X'X for linear models.

Description

postModeOrtho is for diagonal X'X, postModeBlockDiag for the more general block-diagonal X'X, where X is the matrix with predictors.

Both functions return the model of highest posterior probability of any given size using an efficient search algorithm. This sequence of models includes the highest posterior probability model (HPM). Posterior model probabilities, marginal variable inclusion probabilities and Bayesian model averaging estimates are also provided. The unknown residual variance is integrated out using an exact deterministic algorithm of low computational cost (see details in reference).

Usage

postModeOrtho(y, x, priorCoef=momprior(tau=0.348), priorDelta=modelbbprior(1,1),
priorVar=igprior(0.01,0.01), bma=FALSE, includeModels, maxvars=100)

postModeBlockDiag(y, x, blocks, priorCoef=zellnerprior(tau=nrow(x)),
priorDelta=modelbinomprior(p=1/ncol(x)),priorVar=igprior(0.01,0.01), bma=FALSE,
maxvars=100, momcoef)

Arguments

y	Outcome
x	Matrix with predictors. If an intercept is desired x should include a column of 1's.
blocks	Factor or integer vector of length ncol(x) indicating the block that each column in x belongs to.
priorCoef	Prior distribution for the coefficients. Object created with momprior, imomprior, emomprior or zellnerprior.
priorDelta	Prior on model space. Use modelbbprior() for Beta-Binomial prior, modelbinomprior(p) for Binomial prior with prior inclusion probability p, modelcomplexprior for Complexity prior, or modelunifprior() for Uniform prior
priorVar	Inverse gamma prior on residual variance, created with igprior()
bma	Set to TRUE to obtain marginal inclusion probabilities and Bayesian model averaging parameter estimates for each column of x.
includeModels	Models that should always be included when computing posterior model probabilities. It must be a list, each element in the list corresponds to a model and must be a logical or numeric vector indicating the variables in that model
maxvars	The search for the HPM is restricted to models with up to maxvars variables (note: posterior model probabilities and BMA are valid regardless of maxvars)
momcoef	optional argument containing pre-computed coefficients needed to obtain the marginal likelihood under the pMOM prior. A first call to postModeBlockDiag returns these coefficients, thus this argument is useful to speed up successive calls.

Details

The first step is to list a sequence of models with 0,...,maxvars variables which, under fairly general conditions listed in Papaspiliopoulos & Rossell (2016), is guaranteed to include the HPM. Then posterior model probabilities are computed for all these models to determine the HPM, evaluate the marginal posterior of the residual variance on a grid, and subsequently compute the marginal density p(y) via adaptive quadrature. Finally this adaptive grid is used to compute marginal inclusion probabilities and Bayesian model averaging estimates. For more details see Papaspiliopoulos & Rossell (2016).

Value

List with elemants

models	data.frame indicating the variables included in the sequence of models found during the search of the HPM, and their posterior probabilities. The model with highest posterior probability in this list is guaranteed to be the HPM.
phi	data.frame containing an adaptive grid of phi (residual variance) values and their marginal posterior density p(phi|y).
logpy	log-marginal density p(y), i.e. normalization constant of p(phi|y).
bma	Marginal posterior inclusion probabilities and Bayesian model averaging estimates for each column in x.
postmean.model	Coefficient estimates conditional on each of the models in models
momcoef	If a MOM prior was specified in priorCoef, momcoef stores some coefficients needed to compute its marginal likelihood

Author(s)

David Rossell

References

Papaspiliopoulos O., Rossell D. Scalable Bayesian variable selection and model averaging under block-orthogonal design. 2016

Examples

#Simulate data
set.seed(1)
p <- 400; n <- 410
x <- scale(matrix(rnorm(n*p),nrow=n,ncol=p),center=TRUE,scale=TRUE)
S <- cov(x)
e <- eigen(cov(x))
x <- t(t(x %*% e$vectors)/sqrt(e$values))
th <- c(rep(0,p-3),c(.5,.75,1)); phi <- 1
y <- x %*% matrix(th,ncol=1) + rnorm(n,sd=sqrt(phi))

#Fit
priorCoef=zellnerprior(tau=n); priorDelta=modelbinomprior(p=1/p); priorVar=igprior(0.01,0.01)
pm.zell <- postModeOrtho(y,x=x,priorCoef=priorCoef,priorDelta=priorDelta,priorVar=priorVar,
bma=TRUE)

#Best models
head(pm.zell$models)

#Posterior probabilities for sequence of models
nvars <- sapply(strsplit(as.character(pm.zell$models$modelid),split=','),length)
plot(nvars,pm.zell$models$pp,ylab='post prob',xlab='number of vars',ylim=0:1,xlim=c(0,50))

#Marginal posterior of phi
plot(pm.zell$phi,type='l',xlab='phi',ylab='p(phi|y)')

#Marginal inclusion prob & BMA estimates
plot(pm.zell$bma$margpp,ylab='Marginal inclusion prob')
plot(pm.zell$bma$coef,ylab='BMA estimate')

mombf documentation built on May 29, 2024, 11:01 a.m.