Default student_t priors in brms

martinmodrak · August 10, 2020, 1:32pm

The prior is computed in:

paul-buerkner/brms/blob/c2b24475d727c8afd8bfc95947c18793b8ce2892/R/priors.R#L1074


#' @export
def_scale_prior.mvbrmsterms <- function(x, data, ...) {
  out <- ulapply(x$terms, def_scale_prior, data = data, ...)
  names(out) <- x$responses
  out
}

# @param center Should the prior be centererd around zero?
#   If FALSE, the prior location is computed based on Y.
#' @export
def_scale_prior.brmsterms <- function(x, data, center = TRUE, df = 3,
                                      location = 0, scale = 2.5, ...) {
  y <- unname(model.response(model.frame(x$respform, data)))
  link <- x$family$link
  if (has_logscale(x$family)) {
    link <- "log"
  }
  tlinks <- c("identity", "log", "inverse", "sqrt", "1/mu^2")
  if (link %in% tlinks && !is_like_factor(y)) {
    if (link %in% c("log", "inverse", "1/mu^2")) {
      # avoid Inf in link(y)

(for intercept this is called with center = FALSE, and the most relevant part is location_y <- round(median(y_link), 1) )

Does that answer your question?