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I have been doing some research on constrained models and have recently read the paper:

Gunn and Dunson (2005) "A Transformation Approach for Incorporating Monotone or Unimodel Constraints", Biostatistics, 6, 434-449

In this paper they advocate fitting an unconstrained hierarchical model, and then applying the constraint on the posterior distribution. They cite the fact that the usual Gibbs sampling routines (Gelfand et. al., JASA, 1992, 523-532) are difficult to apply to constrained parameter problems when fitting a hierarchical model.

My question is whether JAGS requires this or not, or is it able to implement the constraints in the prior (where I would like them implemented). Suppose I have the following data:

X1 <- c(327,125,7,6,107,277,54)
X2 <- c(637,40,197,36,54,53,97,63,216,118)
N1 <- 7
N2 <- 10

and I want to fit an isotonic regression for set of X values with the hierarchical model:

X1[i] ~ exponential(theta1(i)), i = 1,...,N1
X2[i] ~ exponential(theta2(i)), i = 1,...,N2
theta1[i] ~ exponential(delta1)
theta2[i] ~ exponential(delta2)
delta1 ~ exponential(lambda)
delta2 ~ exponential(lambda)

where lambda is a specified constant, and we add the following constraints:

theta1[1] > theta1[2] > ... > theta1[N1]
theta2[1] > theta2[2] > ... > theta2[N2]

I specified the JAGS model as follows:

model {

     for(i in 1:N1) {
          X1[i] ~ dexp(theta1[i])
          theta10[i] ~ dexp(d1)
     }
     for(i in 1:N2) {
          X2[i] ~ dexp(theta2[i])
          theta20[i] ~ dexp(d2)
     }
     d1 ~ dexp(d0)
     d2 ~ dexp(d0)
     d0 <- 0.01
     theta11[1:N1] <- sort(theta10)
     theta21[1:N2] <- sort(theta20)
     for(i in 1:N1) { 
          theta1[i] <- theta11[N1-i+1]
     }
     for(i in 1:N2) { 
          theta2[i] <- theta21[N2-i+1]
     }
}

JAGS compiles the model and seems to run fine, and the results seem okay. But is it really fitting the model that I think it is fitting?

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  • 1
    $\begingroup$ On the surface, the model seems reasonable. But the right answer is to run a simulation with known "true" parameter values and see if your estimates are far from the truth. See the Cook-Gelman-Rubin framework for doing this. $\endgroup$ – Jack Tanner Mar 29 '12 at 17:34
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There are two questions here: 1) Is JAGS using the correct posterior density function internally; and 2) is JAGS sampling from the correct posterior.

Given your JAGS code, assuming there are no relevant bugs, JAGS is using the correct posterior density internally (i.e.-this is how this kind of constraint is applied in JAGS).

The second question is whether you are actually getting samples from that posterior. That depends on the algorithm which depends on the actual sampler in use (check the JAGS manual, there's a function that will tell you what is in use) and issues like weird/high covariance, posterior tail weight, multi-modality, etc... which are entirely model-specific... as far as I know this model structure results in a nice posterior, but if you want to be sure: 1) plot the posterior; 2) check it analytically; 3) check the model-specific literature; and 4) check by simulation... and 5) plot the posterior. Weird things happen even with simple hierarchical models .

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