model{
  for (i in 1:N) {
	y[i] ~ dbin(p[i],n[i])
	logit(p[i]) <- alpha + beta*(x[i]-x.bar)
	}
  x.bar <- mean(x[])
  alpha ~ dnorm(0.0,1.0E-6)
  beta ~ dnorm(0.0,1.0E-6)
  alpha0 <- alpha - beta*x.bar
  ld50 <- -alpha/beta
}