diff --git a/R/brm_data.R b/R/brm_data.R
index acf195f1..b091df9b 100644
--- a/R/brm_data.R
+++ b/R/brm_data.R
@@ -20,6 +20,8 @@
 #' @param base Character of length 1, name of the baseline response variable.
 #'   Supply `NULL` to ignore or omit.
 #' @param time Character of length 1, name of the discrete time variable.
+#'   Must be a character or factor variable in the data, and the levels
+#'   must be alphanumeric.
 #' @param patient Character of length 1, name of the patient ID variable.
 #' @param covariates Character vector of names of other covariates.
 #' @examples
diff --git a/R/brm_marginal_draws.R b/R/brm_marginal_draws.R
index e012253c..ff14a9e0 100644
--- a/R/brm_marginal_draws.R
+++ b/R/brm_marginal_draws.R
@@ -151,12 +151,20 @@ brm_marginal_draws <- function(
       control = control
     )
   }
+  draws_sigma <- get_draws_sigma(model = model, time = time)
+  draws_effect <- get_draws_effect(
+    draws_difference = draws_difference,
+    draws_sigma = draws_sigma,
+    groups = groups,
+    times = times
+  )
   out <- list()
   out$response <- draws_response
   if (identical(role, "response")) {
     out$change <- draws_change
   }
   out$difference <- draws_difference
+  out$effect <- draws_effect
   out
 }
 
@@ -184,6 +192,24 @@ subtract_control <- function(draws, groups, times, control) {
   out
 }
 
+get_draws_sigma <- function(model, time) {
+  draws <- tibble::as_tibble(posterior::as_draws_df(model))
+  draws <- draws[, grep("^b_sigma_", colnames(draws), value = TRUE)]
+  colnames(draws) <- gsub("^b_sigma_", "", colnames(draws))
+  colnames(draws) <- gsub(paste0("^", time), "", x = colnames(draws))
+  exp(draws)
+}
+
+get_draws_effect <- function(draws_difference, draws_sigma, groups, times) {
+  out <- draws_difference
+  for (group in groups) {
+    for (time in times) {
+      browser()
+    }
+  }
+  out
+}
+
 name_marginal <- function(group, time) {
   sprintf("%s%s%s", group, brm_sep(), time)
 }
diff --git a/R/brm_package.R b/R/brm_package.R
index 332753c5..00c6688a 100644
--- a/R/brm_package.R
+++ b/R/brm_package.R
@@ -32,7 +32,7 @@
 #' @importFrom purrr map_dbl map_df map2_df
 #' @importFrom rlang warn
 #' @importFrom stats as.formula median model.matrix rnorm runif sd
-#' @importFrom tibble tibble
+#' @importFrom tibble as_tibble tibble
 #' @importFrom tidyr expand_grid pivot_longer pivot_wider
 #' @importFrom tidyselect any_of everything
 #' @importFrom trialr rlkjcorr