ppcheck: Posterior Predictive Checks for Bayesian Analyses fit in JAGS

In jagsUI: A Wrapper Around 'rjags' to Streamline 'JAGS' Analyses

Posterior Predictive Checks for Bayesian Analyses fit in JAGS

Description

A simple interface for generating a posterior predictive check plot for a JAGS analysis fit using jagsUI, based on the posterior distributions of discrepency metrics specified by the user and calculated and returned by JAGS (for example, sums of residuals). The user supplies the name of the discrepancy metric calculated for the real data in the argument observed, and the corresponding discrepancy for data simulated by the model in argument simulated. The posterior distributions of the two parameters will be plotted in X-Y space and a Bayesian p-value calculated.

Usage

  pp.check(x, observed, simulated, xlab='Observed data', ylab='Simulated data', 
                     main='Posterior Predictive Check', ...)

Arguments

x	A jagsUI object generated using the jags function
observed	The name of the parameter (as a string) representing the fit of the observed data (e.g. residuals)
simulated	The name of the corresponding parameter (as a string) representing the fit of the new simulated data
xlab	Customize x-axis label
ylab	Customize y-axis label
main	Customize plot title
...	Additional arguments passed to plot.default

Author(s)

Ken Kellner [e-mail].

Examples

  
#Analyze Longley economic data in JAGS
#Number employed as a function of GNP
#See ?jags for a more detailed example

#Get data
data(longley)
gnp <- longley$GNP
employed <- longley$Employed
n <- length(employed)
data <- list(gnp=gnp,employed=employed,n=n)

#Identify filepath of model file
modfile <- tempfile()

#Write model
#Note calculation of discrepancy stats fit and fit.new
#(sums of residuals)
writeLines("
model{

  #Likelihood
  for (i in 1:n){ 

    employed[i] ~ dnorm(mu[i], tau)     
    mu[i] <- alpha + beta*gnp[i]
    
    res[i] <- employed[i] - mu[i]   
    emp.new[i] ~ dnorm(mu[i], tau)
    res.new[i] <- emp.new[i] - mu[i]

  }
    
  #Priors
  alpha ~ dnorm(0, 0.00001)
  beta ~ dnorm(0, 0.00001)
  sigma ~ dunif(0,1000)
  tau <- pow(sigma,-2)
  
  #Derived parameters
  fit <- sum(res[])
  fit.new <- sum(res.new[])

}
", con=modfile)

#Set parameters to monitor
params <- c('alpha','beta','sigma','fit','fit.new')

#Run analysis

out <- jags(data = data,
            inits = NULL,
            parameters.to.save = params,
            model.file = modfile,
            n.chains = 3,
            n.adapt = 100,
            n.iter = 1000,
            n.burnin = 500,
            n.thin = 2)

#Examine output summary

out

#Posterior predictive check plot

pp.check(out, observed = 'fit', simulated = 'fit.new')

jagsUI documentation built on May 29, 2024, 9:59 a.m.