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I try to implement a spatial intrinsic Gaussian Markov random field model (order = 1) in openBugs via the car.normal() distribution. For a regular 3x3 grid it works fine. But if I increase the sample size (say 4x4) I get an error. Why? I am grateful for hints... 

Here is a minimal example, that does not work for 4x4 (but for 3x3 it does): 

The file "model.txt":

    model{
	   # likelihoood
       for(i in 1:No){ Y[i] ~ dnorm(Z[i], tau) } 
       
       Z[1:No] ~ car.normal(adj[], weights[], num[], kappa)
       for(k in 1:Nw) { weights[k] <- 1 }      

       # prior
       kappa ~ dgamma(1, 1) 
       tau ~ dgamma(1, 1) 
    }

And the "data.txt" for a 4x4 grid:
    
    list(No = 16,
    Y = c(0.67,0.88,0.38,0.1,0.78,0.67,0.25,0.55,0.11,0.85,0.46,0.5,0.47,0.38,0.21,0.97), 
    adj = c( 
    5,2,
    6,1,3,
    7,2,4,
    8,3,
    1,9,6,
    2,10,5,7,
    6,14,9,11,
    4,12,7,
    5,13,10,
    3,11,6,8,
    7,15,10,12,
    8,16,11,
    9,14,
    10,13,15,
    11,14,16,
    12,15),
    num = c(2,3,3,2,3,4,4,3,3,4,4,3,2,3,3,2),
    Nw = 48)

I run it in R (other options are possible):
  
    require(R2OpenBUGS)
    bugs(data="data.txt",
         inits=function(){list(tau=10, kappa=100, Z = rep(c(-.5,.5),8))},
         parameters=c("Z","tau","kappa"),
         model.file="model.txt",
         n.chains=1, n.iter=150, n.burnin=1)


The error is:

    OpenBUGS version 3.2.2 rev 1063
    model is syntactically correct
    data loaded
    model compiled
    error for node Z[1] of type SpatialCARNormal.Node first argument Graph18 error pos 260 (error on line 1)
    error for node Z[1] of type SpatialCARNormal.Node first argument Graph18
    model must be initialized before updating
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before monitors used
    model must be initialized before DIC can be monitored
    model must be initialized before updating
    model must be initialized before monitors used
    model must be initialized before monitors used
    DIC monitor not set

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