Hello

I suspect this is a basic error I am making but can’t see where I am going wrong. I am trying to run a hierarchical model with a covariance matrix based on Richard McElreath’s code.However I keep getting this error message:

SAMPLING FOR MODEL ‘e3ad553ffed76cee7e2a7a541857f7ba’ NOW (CHAIN 1).

Unrecoverable error evaluating the log probability at the initial value.

Exception: []: accessing element out of range. index 436 out of range; expecting index to be between 1 and 256; index position = 1a_group (in ‘model28341223245f_e3ad553ffed76cee7e2a7a541857f7ba’ at line 42)

[1] “Error in sampler$call_sampler(args_list[[i]]) : "

[2] " Exception: []: accessing element out of range. index 436 out of range; expecting index to be between 1 and 256; index position = 1a_group (in ‘model28341223245f_e3ad553ffed76cee7e2a7a541857f7ba’ at line 42)”

[1] “error occurred during calling the sampler; sampling not done”. The dataset is simply looking at the effect of age on weight in children with food allergies. My code:allergy_data.csv (25.8 KB)

My code:

```
model_string<-
"data {
int<lower=1> N; // Number of observations
int<lower=1> N_group; // Number of groups
real y[N]; // outcome
int group[N]; // group label
real age[N]; // predictor
}
parameters {
vector[N_group] b_group;
vector[N_group] a_group;
real a;
real b;
vector<lower=0>[2] sigma_group;
real<lower=0> sigma;
corr_matrix[2] Omega;
}
transformed parameters {
vector[2] vec_ab[N_group];
vector[2] Mu_ab;
for (j in 1:N_group) {
vec_ab[j, 1] = a_group[j];
vec_ab[j, 2] = b_group[j];
}
for (j in 1:2) {
Mu_ab[1] = a;
Mu_ab[2] = b;
}
}
model {
Omega ~ lkj_corr(2); // Prior for correlation matrix
sigma ~ cauchy(0, 2.5); // Prior std dev within groups
sigma_group ~ cauchy(0, 2.5); // Prior std dev among intercepts and slopes
b ~ normal(0, 10); // Prior for average slope
a ~ normal(mean(y), 10); // Prior for average intercept
vec_ab ~ multi_normal(Mu_ab, quad_form_diag(Omega, sigma_group)); # Population of varying effects
{
vector[N] mu;
for (i in 1:N) { // Linear model
mu[i] = a_group[group[i]] + b_group[group[i]] * age[i];
}
y ~ normal(mu, sigma); // Likelihood
}
}
"
data_list<-list(N=length(allergy$weight),y=allergy$weight,age=allergy$age,N_group=length(unique(allergy$idnum)),group=allergy$idnum)
```

I think it may have something to do with the way I am trying to pass the data to Stan,but I can’t see the problem. It’s pointing to where it wants to sample mu for the first time.

Thanks for any advice

Regards

Chris