diff --git a/R/brm_marginal_draws.R b/R/brm_marginal_draws.R
index 57903828..1cf755da 100644
--- a/R/brm_marginal_draws.R
+++ b/R/brm_marginal_draws.R
@@ -6,8 +6,8 @@
 #'   distribution of each treatment group and time point:
 #'   * `response`: on the scale of the response variable.
 #'   * `change`: change from baseline, where the `baseline` argument determines
-#'     the time point at baseline. Only returned if the `outcome` argument is
-#'     `"response"`. (If `outcome` is `"change"`, then `response` already
+#'     the time point at baseline. Only returned if the `role` argument is
+#'     `"response"`. (If `role` is `"change"`, then `response` already
 #'     represents change from baseline.)
 #'   * `difference`: treatment effect of change from baseline, where the
 #'     `control` argument identifies the placebo or active control group.
@@ -15,12 +15,8 @@
 #'   each type of marginal distribution (i.e. each combination of treatment
 #'   group and discrete time point.
 #'   Treatment and time are comma-delimited in the column names.
-#' @inheritParams brm_formula
 #' @param model Fitted `brms` model object from [brm_model()].
-#' @param outcome Character of length 1, `"response"` if the
-#'   response variable is the raw outcome variable (such as AVAL)
-#'   or `"change"` if the response variable is change from baseline
-#'   (e.g. CHG).
+#' @param data Classed tibble from [brm_data()].
 #' @param control Element of the `group` column in the data which indicates
 #'   the control group for the purposes of calculating treatment differences.
 #' @param baseline Element of the `time` column in the data
@@ -28,15 +24,18 @@
 #'   change from baseline.
 #' @examples
 #' set.seed(0L)
-#' sim <- brm_simulate()
-#' data <- sim$data
+#' data <- brm_data(
+#'   data = tibble::as_tibble(brm_simulate()$data),
+#'   outcome = "response",
+#'   role = "response",
+#'   group = "group",
+#'   time = "time",
+#'   patient = "patient"
+#' )
 #' data$group <- paste("treatment", data$group)
 #' data$time <- paste("visit", data$time)
 #' formula <- brm_formula(
-#'   response = "response",
-#'   group = "group",
-#'   time = "time",
-#'   patient = "patient",
+#'   data = data,
 #'   effect_base = FALSE,
 #'   interaction_base = FALSE
 #' )
@@ -55,37 +54,23 @@
 #' )
 #' brm_marginal_draws(
 #'   model = model,
-#'   group = "group",
-#'   time = "time",
-#'   patient = "patient",
+#'   data = data,
 #'   control = "treatment 1",
-#'   baseline = "visit 1",
-#'   outcome = "response"
+#'   baseline = "visit 1"
 #' )
 brm_marginal_draws <- function(
   model,
-  base = "BASE",
-  group = "TRT01P",
-  time = "AVISIT",
-  patient = "USUBJID",
-  covariates = character(0),
-  outcome = "change",
+  data,
   control = "Placebo",
   baseline = "Baseline"
 ) {
-  assert_chr(base, "base arg must be a nonempty character string")
-  assert_chr(group, "group arg must be a nonempty character string")
-  assert_chr(time, "time arg must be a nonempty character string")
-  assert_chr(patient, "patient arg must be a nonempty character string")
-  assert_chr(
-    outcome,
-    "outcome arg must be a nonempty character string"
-  )
-  assert(
-    outcome %in% c("response", "change"),
-    message = "outcome must be either \"response\" or \"change\""
-  )
-  assert_chr_vec(covariates, "covariates arg must be a character vector")
+  brm_data_validate(data)
+  role <- attr(data, "role")
+  base <- attr(data, "base")
+  group <- attr(data, "group")
+  time <- attr(data, "time")
+  patient <- attr(data, "patient")
+  covariates <- attr(data, "covariates")
   assert(
     control,
     is.atomic(.),
@@ -100,28 +85,16 @@ brm_marginal_draws <- function(
     !anyNA(.),
     message = "baseline arg must be a length-1 non-missing atomic value"
   )
-  assert(is.data.frame(model$data))
-  data <- model$data
-  assert(
-    group %in% colnames(data),
-    message = "group arg must be a data column name"
-  )
-  assert(
-    time %in% colnames(data),
-    message = "time arg must be a data column name"
-  )
-  assert(
-    patient %in% colnames(data),
-    message = "patient arg must be a data column name"
-  )
-  assert(
-    covariates %in% colnames(data),
-    message = "all covariates must be data column names"
-  )
   assert(
     control %in% data[[group]],
-    message = "control arg must be in data[[group]]"
+    message = "control arg must be a treatment group level in the data"
   )
+  if (identical(role, "response")) {
+    assert(
+      baseline %in% data[[time]],
+      message = "control arg must be a discrete time level in the data"
+    )
+  }
   nuisance <- c(base, patient, covariates)
   emmeans <- emmeans::emmeans(
     object = model,
@@ -143,7 +116,7 @@ brm_marginal_draws <- function(
       paste(groups, collapse = ", ")
     )
   )
-  if (identical(outcome, "response")) {
+  if (identical(role, "response")) {
     assert(
       baseline %in% times,
       message = sprintf(
@@ -174,7 +147,7 @@ brm_marginal_draws <- function(
   }
   out <- list()
   out$response <- draws_response
-  if (identical(outcome, "response")) {
+  if (identical(role, "response")) {
     out$change <- draws_change
   }
   out$difference <- draws_difference
diff --git a/man/brm_marginal_draws.Rd b/man/brm_marginal_draws.Rd
index 8f01d0aa..2950bf62 100644
--- a/man/brm_marginal_draws.Rd
+++ b/man/brm_marginal_draws.Rd
@@ -4,25 +4,12 @@
 \alias{brm_marginal_draws}
 \title{MCMC draws from the marginal posterior of an MMRM}
 \usage{
-brm_marginal_draws(
-  model,
-  base = "BASE",
-  group = "TRT01P",
-  time = "AVISIT",
-  patient = "USUBJID",
-  covariates = character(0),
-  outcome = "change",
-  control = "Placebo",
-  baseline = "Baseline"
-)
+brm_marginal_draws(model, data, control = "Placebo", baseline = "Baseline")
 }
 \arguments{
 \item{model}{Fitted \code{brms} model object from \code{\link[=brm_model]{brm_model()}}.}
 
-\item{outcome}{Character of length 1, \code{"response"} if the
-response variable is the raw outcome variable (such as AVAL)
-or \code{"change"} if the response variable is change from baseline
-(e.g. CHG).}
+\item{data}{Classed tibble from \code{\link[=brm_data]{brm_data()}}.}
 
 \item{control}{Element of the \code{group} column in the data which indicates
 the control group for the purposes of calculating treatment differences.}
@@ -37,8 +24,8 @@ distribution of each treatment group and time point:
 \itemize{
 \item \code{response}: on the scale of the response variable.
 \item \code{change}: change from baseline, where the \code{baseline} argument determines
-the time point at baseline. Only returned if the \code{outcome} argument is
-\code{"response"}. (If \code{outcome} is \code{"change"}, then \code{response} already
+the time point at baseline. Only returned if the \code{role} argument is
+\code{"response"}. (If \code{role} is \code{"change"}, then \code{response} already
 represents change from baseline.)
 \item \code{difference}: treatment effect of change from baseline, where the
 \code{control} argument identifies the placebo or active control group.
@@ -53,15 +40,18 @@ Get marginal posterior draws from a fitted MMRM.
 }
 \examples{
 set.seed(0L)
-sim <- brm_simulate()
-data <- sim$data
+data <- brm_data(
+  data = tibble::as_tibble(brm_simulate()$data),
+  outcome = "response",
+  role = "response",
+  group = "group",
+  time = "time",
+  patient = "patient"
+)
 data$group <- paste("treatment", data$group)
 data$time <- paste("visit", data$time)
 formula <- brm_formula(
-  response = "response",
-  group = "group",
-  time = "time",
-  patient = "patient",
+  data = data,
   effect_base = FALSE,
   interaction_base = FALSE
 )
@@ -80,12 +70,9 @@ tmp <- utils::capture.output(
 )
 brm_marginal_draws(
   model = model,
-  group = "group",
-  time = "time",
-  patient = "patient",
+  data = data,
   control = "treatment 1",
-  baseline = "visit 1",
-  outcome = "response"
+  baseline = "visit 1"
 )
 }
 \seealso{
diff --git a/tests/testthat/test-brm_marginal_draws.R b/tests/testthat/test-brm_marginal_draws.R
index 81441626..3b477af3 100644
--- a/tests/testthat/test-brm_marginal_draws.R
+++ b/tests/testthat/test-brm_marginal_draws.R
@@ -1,18 +1,17 @@
 test_that("brm_marginal_draws() on response", {
   set.seed(0L)
-  sim <- brm_simulate(
-    n_group = 2L,
-    n_patient = 100L,
-    n_time = 4L
+  data <- brm_data(
+    data = tibble::as_tibble(brm_simulate()$data),
+    outcome = "response",
+    role = "response",
+    group = "group",
+    time = "time",
+    patient = "patient"
   )
-  data <- sim$data
   data$group <- paste("treatment", data$group)
   data$time <- paste("visit", data$time)
   formula <- brm_formula(
-    response = "response",
-    group = "group",
-    time = "time",
-    patient = "patient",
+    data = data,
     effect_base = FALSE,
     interaction_base = FALSE
   )
@@ -31,12 +30,9 @@ test_that("brm_marginal_draws() on response", {
   )
   out <- brm_marginal_draws(
     model = model,
-    group = "group",
-    time = "time",
-    patient = "patient",
+    data = data,
     control = "treatment 1",
-    baseline = "visit 1",
-    outcome = "response"
+    baseline = "visit 1"
   )
   fields <- c("response", "change", "difference")
   columns_df <- expand.grid(
@@ -93,19 +89,18 @@ test_that("brm_marginal_draws() on response", {
 
 test_that("brm_marginal_draws() on change", {
   set.seed(0L)
-  sim <- brm_simulate(
-    n_group = 2L,
-    n_patient = 100L,
-    n_time = 4L
+  data <- brm_data(
+    data = tibble::as_tibble(brm_simulate()$data),
+    outcome = "response",
+    role = "change",
+    group = "group",
+    time = "time",
+    patient = "patient"
   )
-  data <- sim$data
   data$group <- paste("treatment", data$group)
   data$time <- paste("visit", data$time)
   formula <- brm_formula(
-    response = "response",
-    group = "group",
-    time = "time",
-    patient = "patient",
+    data = data,
     effect_base = FALSE,
     interaction_base = FALSE
   )
@@ -124,12 +119,9 @@ test_that("brm_marginal_draws() on change", {
   )
   out <- brm_marginal_draws(
     model = model,
-    group = "group",
-    time = "time",
-    patient = "patient",
+    data = data,
     control = "treatment 1",
-    baseline = "visit 1",
-    outcome = "change"
+    baseline = "visit 1"
   )
   fields <- c("response", "difference")
   columns_df <- expand.grid(