Hi - I’m using rstanarm and stan_glmer to run a a multilevel beta regression, but I can’t work out how to set a prior for phi. In the stan_betareg function you can specify prior_phi, but that doesn’t seem to be the case for the stan_glmer with family=mgcv::betar instance.

In addition - I can’t work out what the current default prior for phi is:

- Running prior_summary on the model doesn’t show a prior for phi.
- pp_validate throws a “Error in exp(eta) : non-numeric argument to mathematical function” that I assume is related to the error for posterior_predict (documented here Rstanarm: mgcv::betar family).

Any suggestions on how to proceed?