# Undefined real result error at WinBUGS

I am currently working on my thesis and interested in estimating a multilevel differential item functioning model and I using at WinBUGS. Until I had done model check-up, there are no errors. However, when I tried to "update" the sample, suddenly the trap message said "undefined real result" popped up. What am I doing wrong? Also, I have tried different prior and initial values but I could not solve the problem!

   # Model
Model
{
for (l in 1:10){
y[l] ~ dbern(p[l])
logit(p[l])<- u2[stu[l]] - beta[x[l]] + gamma[tea[l], x[l]]*grp[l] + alpha1[x[l]] *geo[l] +
alpha2[x[l]]*conf[l] + alpha3[x[l]]*ses[l]
}
for (t in 1:5){
for (i in 1:5){
gamma[t,i] ~ dnorm(gamma.hat[t,i], tau.gamma[i])
gamma.hat[t,i]<-pi1[i] + pi2[i]*inq[t]
}
}
# fixed effect prior
for (i in 1:5){
beta[i] ~ dnorm(0, .0001)
alpha1[i] ~ dnorm(0, .0001)
alpha2[i] ~ dnorm(0, .0001)
alpha3[i] ~ dnorm(0, .0001)
pi1[i] ~ dnorm(0, .0001)
pi2[i] ~ dnorm(0, .0001)
}
# Random effect prior
for (s in 1:10){
u2[s] ~ dnorm(0,tau.u2)
}
tau.u2 <- pow(sigma.u2, -2)
sigma.u2 ~ dunif (0, 100)
for (i in 1:5){
tau.gamma[i] <- pow(sigma.gamma[i],-2)
sigma.gamma[i] ~ dunif(0, 100)
}
}

# Data
list(y=c(0, 1, 1, 0, 0, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0,  1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 1, 0), ses=c(2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1), conf=c(1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3), geo=c(3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 3, 3, 3), grp=c(1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0), inq=c(1, 2, 2, 1, 2), stu=c(1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10), x=c(1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5), tea=c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5))

#Initital values
list(beta=c(0, 0, 0, 0, 0), alpha1=c(0, 0, 0, 0, 0),  alpha2=c(0, 0, 0, 0, 0),  alpha3=c(0, 0, 0, 0, 0), sigma.gamma=c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1), u2=c(0, 0, 0, 0, 0), pi1=c(0, 0, 0, 0, 0), pi2=c(0, 0, 0, 0, 0), sigma.u2=1, gamma=structure(
.Data=c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0), .Dim=c(5, 5)))

• @gung BUGS (and WinBUGS) differ from other coding platforms in that (a) they are dedicated to running statistical models (and cannot run general-purpose code) and (b) frequently errors result from mis-specifying or misunderstanding those models, rather than from errors of syntax or organization of code. Chris Jackson's answer provides an example of what kind of help might be useful. It suggests to me that this question should be considered on topic here. – whuber Oct 22 '14 at 23:15
• @whuber, fair enough. I have retracted my close vote. – gung - Reinstate Monica Oct 22 '14 at 23:37