# [R-sig-ME] BUGS/JAGS: prior for a 2D random effect

Andrew Digby andrewdigby at mac.com
Thu Nov 1 06:28:23 CET 2012

```I'm writing a hierarchical BUGS/JAGS model with two 'for' loops, and want to include nested random effects, a1 & a2 (nested in a1). But I can't work out how to specify the prior for the nested random effect. This doesn't work:

model{
for (i in 1:N){
for (j in 1:M){
...
eta[j,i] <- alpha + b*Var1[j,i] + ... + a1[i] + a2[j,i]
}
}

for (i in 1:N){a1[i] ~ dnorm(0.0, tau1}    # Prior for random effect a1
# This next part, specifying the prior for random effect a2, is wrong, but what should it be:?
for (i in 1:N){
for (j in 1:M){
a2[j, i] ~ dnorm(0.0, tau2}
}
}
}

inits <- function(){
list ( ...
a1 = rnorm(N, 0, 1),
a2 = matrix(rnorm(N*M, 0, 1), nrow=M, ncol=N),
...
)
}

Apologies for the lack of reproducible example, but this is just a syntax issue. How do I specify the prior for the nested random effect a2? I can't find the answer to this anywhere. I know I can reduce this to just one 'for' loop, but don't want to for other reasons (I later want to introduce an AR1 structure within one loop).

Thanks very much,

Andrew

```