Short summary of the problem

Hi there,

I was trying to fit a negative binomial model with rbms and kept receiving such warning messages,

“Chain 4 Informational Message: The current Metropolis proposal is about to be rejected because of the following issue:

Chain 4 Exception: neg_binomial_2_log_glm_lpmf: Precision parameter is 0, but must be positive finite! (in ‘/scratch/tmp/RtmplN2FN4/model-1a00d017791c7d.stan’, line 37, column 4 to column 65)

Chain 4 If this warning occurs sporadically, such as for highly constrained variable types like covariance matrices, then the sampler is fine,

Chain 4 but if this warning occurs often then your model may be either severely ill-conditioned or misspecified.” Even after adjusting control=list(adapt_delta=0.999), the issue remains, can anyone help me fixing this?

Another question is what’s the difference between using “offset(log(time))” and “counts|rate(time)”?

Thank you very much in advance for your reply!

code_to_run_your_model(if_applicable)

```
model1 <- bf(counts ~ 1 + period + arm:period + baseline + offset(log(time)), family = negbinomial("log"))
prior1 <- get_prior(model1, dd)
brmfit1 <- brm(model1,
data=dd,
family = negbinomial("log"),
prior = prior1,
control=list(adapt_delta=0.999),
refresh = 0, silent = 0, seed = 0312)
```