Hello,

First time poster. I am trying to fit a multinomial regression model in RStan. I have species richness counts/outcome value (1-8) and vegetation types (1-14) from 645 sites. I am trying to estimate the probability of species richness count for each vegetation type. However when I try to run Stan I get this error returned:

SAMPLING FOR MODEL ‘6b9f964743cbd2e39ffe150107d8dddf’ NOW (CHAIN 1).

Chain 1: Unrecoverable error evaluating the log probability at the initial value.

Chain 1: Exception: []: accessing element out of range. index 8 out of range; expecting index to be between 1 and 7; index position = 1b_veg (in ‘model84e0b6f6c72_6b9f964743cbd2e39ffe150107d8dddf’ at line 15)

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

[2] " Exception: []: accessing element out of range. index 8 out of range; expecting index to be between 1 and 7; index position = 1b_veg (in ‘model84e0b6f6c72_6b9f964743cbd2e39ffe150107d8dddf’ at line 15)”

[3] “In addition: Warning message:”

[4] “In system(paste(CXX, ARGS), ignore.stdout = TRUE, ignore.stderr = TRUE) :”

[5] " ‘-E’ not found"

I’m really at a loss for determining what is wrong. My index should be 8, because there are 8 species richness counts. Am I missing something?

Here is what my data looks like:

```
List of 4
$ N : num 645
$ K : num 8
$ rich: num [1:645] 3 3 6 1 3 5 7 6 7 4 ...
$ veg : num [1:645] 5 8 14 10 12 7 8 11 14 9 ...
data{
int N; // number of observations
int K; // number of outcome values
int rich[N]; // species richness
int veg[N]; // veg code
}
parameters{
vector[K-1] b_veg; // intercepts
}
model{
b_veg ~ normal(0,1); // prior
for ( i in 1:N ) {
vector[K] p;
for ( j in 1:(K-1) ) p[j] = b_veg[veg[i]];
p[K] = 0; // the pivot
rich[i] ~ categorical_logit( p );
}
}
```

Thanks for your help in advance.

Alan