for model comparison, I want to calculate Bayes factors. The models I ran with brms are ordinal models that allow for unequal variance by including the disc parameter. It seems to not be sensible to use the formula for unequal variances, because there are no different groups in the null model. Do I understand it correctly, that I can simply compare the models to the null model of the respective model with equal variances? Or do I have to fit the null model using the bf() and lf() formulas? If the latter is the case, could you maybe show me how this null model might look like for, e.g. the following model? Thank you!
fit3 <- brm(data = data, family = cumulative(probit), bf(Y ~ 1 + group) + lf(disc ~ 0 + group, cmc = FALSE), prior = c(prior(normal(0, 4), class = Intercept), prior(normal(0, 2), class = b), prior(normal(0, 1), class = b, dpar = disc))