m12.1

[ASKurz]

In Section 12.1, you opened with a declaration you haven't had success with m12.. I think I've figured it out here: https://solomon.quarto.pub/sr2rstan/12.html#sec-Beta-binomial. Here's the relevant model_code:

model_code_12.1 <- '
data {
  int<lower=1> n;                       
  int<lower=1> n_gid;                       
  array[n] int applications;
  array[n] int gid;
  array[n] int admit;
}
parameters {
  vector[n_gid] a;
  real<lower=0> phi;
}
transformed parameters {
  real theta;
  theta = phi + 2;
  
  vector[n] pbar;
  pbar = inv_logit(a[gid]);
}
model {
  vector[n] alpha;
  vector[n] beta;
  alpha = pbar * theta;
  beta = (1 - pbar) * theta;
  
  admit ~ beta_binomial(applications, alpha, beta);
  a ~ normal(0, 1.5);
  phi ~ exponential(1);
}
'