Hello:

I have been pondering about running a simple interaction between two categorical variables using `rethinking::ulam()`

function for measuring weight changes between two treatments (treatment vs. control). Time points are recorded as factor type.

The fit function is quite simple using `nlme`

package

```
lme(bmi~ trmt*time_pts+bmi_baseline,
random = ~1|ID,
data=df.long,
na.action = na.omit)
```

Translating this simple linear mixed effect model to `rethinking::ulam()`

, I wrote the following code:

```
dat<- list(
B=df.long$bmi,
bm=df.long$bmi_baseline,
T=df.long$ID,
P=case_when(df.long$time_pts ==1 ~1L,
df.long$time_pts ==2 ~2L,
df.long$time_pts ==3 ~3L,),
G= ifelse(df.long$trmt == 1, 1L, 2L)
)
fit.stan1<- ulam(
alist(
B ~ normal(mu, sigma),
mu <- a+ b[P,G]+ bm*bm +c[T],
a ~ dnorm(33, 5),
bm~dnorm(0,5),
sigma~ dexp(1),
#adaptive prior
matrix[P,G]:b ~ dnorm(0,sigma_a),
c[IDf] ~ normal (bbar, sigma_bar),
#hyper-priors
sigma_a ~ dexp(1),
bbar ~ normal (0,5),
sigma_bar ~ dexp(1)
),
data=dat,
seed=2022,
chains = 2,
iter = 2000,
warmup = 1000
)
```

I believe I specified the model correctly, but it shows the following error message:

## Warning: No chains finished successfully. Unable to retrieve the fit.Error in Stan::read_stan_csv(cmdstanfit$output_files()) : csvfiles does not contain any CSV file name

I am not sure what that error message is and how to trouble shoot it.

Any guidance on this matter would be greatly appreciated.