I am fitting a phylogenetic model following the instructions here: https://cran.r-project.org/web/packages/brms/vignettes/brms_phylogenetics.html. Something like this:
b.1 <- brm(Y ~ X + (1 | phylo) , data = df , family = poisson , cov_ranef = list(phylo = phylo))
Now, I am splitting the data into a training and testing data sets, but when I am trying to make the predictions for the testing dataset brm complains that there are new levels in the random effects. However, the phylo object contains those new levels, and they are in the training dataset as factors. Maybe I am misunderstanding something, but why can’t the model make use of this information?