diff --git a/R/brm_summary.R b/R/brm_summary.R
index 2d1bb4d5..e7722fb8 100644
--- a/R/brm_summary.R
+++ b/R/brm_summary.R
@@ -7,12 +7,21 @@
 #' @return A `tibble` with summary statistics of the marginal posterior.
 #' @inheritParams brm_formula
 #' @param model Fitted `brms` model object from [brm_model()].
-#' @param control Character of length 1, name of the control arm
-#'   in the `group` column in the 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
+#'   which indicates the baseline time for the purposes of calculating
+#'   change from baseline. Ignored if `response_type = "change"`.
+#' @param response_type 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).
 #' @examples
 #' set.seed(0L)
 #' sim <- brm_simulate()
 #' data <- sim$data
+#' data$group <- paste("treatment", data$group)
+#' data$time <- paste("visit", data$time)
 #' formula <- brm_formula(
 #'   response = "response",
 #'   group = "group",
@@ -39,7 +48,9 @@
 #'   group = "group",
 #'   time = "time",
 #'   patient = "patient",
-#'   control = 1
+#'   control = "treatment 1",
+#'   baseline = "visit 1",
+#'   response_type = "response"
 #' )
 brm_summary <- function(
   model,
@@ -48,12 +59,22 @@ brm_summary <- function(
   time = "AVISIT",
   patient = "USUBJID",
   covariates = character(0),
-  control = "Placebo"
+  control = "Placebo",
+  baseline = "Baseline",
+  response_type = "change"
 ) {
   assert_chr(base, "base arg must be a nonempty character string")
   assert_chr(group, "group arg must be a nonempty character string")
   assert_chr(time, "time arg must be a nonempty character string")
   assert_chr(patient, "patient arg must be a nonempty character string")
+  assert_chr(
+    response_type,
+    "response_type arg must be a nonempty character string"
+  )
+  assert(
+    response_type %in% c("response", "change"),
+    message = "response_type must be either \"response\" or \"change\""
+  )
   assert_chr_vec(covariates, "covariates arg must be a character vector")
   assert(
     control,
@@ -62,6 +83,13 @@ brm_summary <- function(
     !anyNA(.),
     message = "control arg must be a length-1 non-missing atomic value"
   )
+  assert(
+    baseline,
+    is.atomic(.),
+    length(.) == 1L,
+    !anyNA(.),
+    message = "baseline arg must be a length-1 non-missing atomic value"
+  )
   assert(is.data.frame(model$data))
   data <- model$data
   assert(
@@ -85,82 +113,104 @@ brm_summary <- function(
     message = "control arg must be in data[[group]]"
   )
   nuisance <- c(base, patient, covariates)
-  emmeans_response <- emmeans::emmeans(
+  emmeans <- emmeans::emmeans(
     object = model,
-    specs = as.formula(sprintf("~%s:%s", time, group)),
+    specs = as.formula(sprintf("~%s:%s", group, time)),
     weights = "proportional",
     nuisance = nuisance
   )
-  table_response <- brm_summary_response(
-    data = data,
-    emmeans_response = emmeans_response
+  draws_response <- posterior::as_draws_df(as.mcmc(emmeans))
+  .chain <- draws_response[[".chain"]]
+  .iteration <- draws_response[[".iteration"]]
+  .draw <- draws_response[[".draw"]]
+  draws_response[[".chain"]] <- NULL
+  draws_response[[".iteration"]] <- NULL
+  draws_response[[".draw"]] <- NULL
+  colnames(draws_response) <- gsub(
+    pattern = sprintf("^%s ", group),
+    replacement = "",
+    x = colnames(draws_response)
   )
-  table_diff <- brm_summary_diff(
-    data = data,
-    emmeans_response = emmeans_response,
-    group = group,
-    time = time,
-    nuisance = nuisance,
-    control = control
+  colnames(draws_response) <- gsub(
+    pattern = sprintf(", %s ", time),
+    replacement = ", ",
+    x = colnames(draws_response)
   )
-  dplyr::left_join(
-    x = table_response,
-    y = table_diff,
-    by = c(group, time)
+  groups <- unique(gsub(",.*$", "", colnames(draws_response)))
+  times <- unique(gsub("^.*, ", "", colnames(draws_response)))
+  draws_response[[".chain"]] <- .chain
+  draws_response[[".iteration"]] <- .iteration
+  draws_response[[".draw"]] <- .draw
+  control <- as.character(control)
+  time <- as.character(time)
+  assert(
+    control %in% groups,
+    message = sprintf(
+      "control argument \"%s\" is not in one of the treatment groups: %s",
+      control,
+      paste(groups, collapse = ", ")
+    )
   )
+  if (response_type == "response") {
+    assert(
+      baseline %in% times,
+      message = sprintf(
+        "baseline argument \"%s\" is not in one of the time points: %s",
+        baseline,
+        paste(times, collapse = ", ")
+      )
+    )
+  }
+  if (response_type == "response") {
+    draws_change <- subtract_baseline(
+      draws = draws_response,
+      groups = groups,
+      times = times,
+      baseline = baseline
+    )
+    draws_diff <- subtract_control(
+      draws = draws_change,
+      groups = groups,
+      times = setdiff(times, baseline),
+      control = control
+    )
+  } else {
+    draws_diff <- subtract_control(
+      draws = draws_response,
+      groups = groups,
+      times = times,
+      control = control
+    )
+  }
+  
+  browser()
+  
 }
 
-brm_summary_response <- function(data, emmeans_response) {
-  out <- tibble::as_tibble(emmeans_response)
-  out$response_mean <- out$emmean
-  out$response_lower <- out$lower.HPD
-  out$response_upper <- out$upper.HPD
-  out$emmean <- NULL
-  out$lower.HPD <- NULL
-  out$upper.HPD <- NULL
+subtract_baseline <- function(draws, groups, times, baseline) {
+  out <- draws[, c(".chain", ".iteration", ".draw")]
+  for (group in groups) {
+    for (time in setdiff(times, baseline)) {
+      name1 <- marginal_name(group, baseline)
+      name2 <- marginal_name(group, time)
+      out[[name2]] <- draws[[name2]] - draws[[name1]]
+    }
+  }
   out
 }
 
-brm_summary_diff <- function(
-  data,
-  emmeans_response,
-  group,
-  time,
-  nuisance,
-  control
-) {
-  reference <- tibble::as_tibble(emmeans_response)
-  contrasts_diff <- list()
-  for (level_group in setdiff(sort(unique(reference[[group]])), control)) {
-    for (level_time in sort(unique(reference[[time]]))) {
-      contrast_treatment <- as.integer(
-        (reference[[group]] == level_group) &
-          (reference[[time]] == level_time)
-      )
-      contrast_control <- as.integer(
-        (reference[[group]] == control) &
-          (reference[[time]] == level_time)
-      )
-      contrast <- contrast_treatment - contrast_control
-      contrasts_diff[[length(contrasts_diff) + 1L]] <- contrast
+subtract_control <- function(draws, groups, times, control) {
+  out <- draws[, c(".chain", ".iteration", ".draw")]
+  for (group in setdiff(groups, control)) {
+    for (time in times) {
+      name1 <- marginal_name(control, time)
+      name2 <- marginal_name(group, time)
+      out[[name2]] <- draws[[name2]] - draws[[name1]]
     }
   }
-  emmeans_diff <- emmeans::contrast(
-    emmeans_response,
-    method = contrasts_diff,
-    adjust = "none",
-    nuisance = nuisance
-  )
-  subset_reference <- reference[reference[[group]] != control, ]
-  out <- tibble::as_tibble(emmeans_diff)
-  out[[group]] <- subset_reference[[group]]
-  out[[time]] <- subset_reference[[time]]
-  out$contrast <- NULL
-  out$diff_mean <- out$estimate
-  out$diff_lower <- out$lower.HPD
-  out$diff_upper <- out$upper.HPD
-  out$estimate <- NULL
-  out$lower.HPD <- NULL
-  out$upper.HPD <- NULL
   out
 }
+
+marginal_name <- function(group, time) {
+  sprintf("%s, %s", group , time)
+}
diff --git a/R/utils_assert.R b/R/utils_assert.R
index 7b2de1a9..1bb5efa6 100644
--- a/R/utils_assert.R
+++ b/R/utils_assert.R
@@ -33,7 +33,7 @@ assert_chr_vec <- function(value, message = NULL) {
 
 assert_chr <- function(value, message = NULL) {
   assert_chr_vec(value, message = message)
-  assert(value, length(.) == 1L)
+  assert(value, length(.) == 1L, message = message)
 }
 
 assert_lgl <- function(value, message = NULL) {
diff --git a/man/brm_summary.Rd b/man/brm_summary.Rd
index fa464928..3f68d289 100644
--- a/man/brm_summary.Rd
+++ b/man/brm_summary.Rd
@@ -11,7 +11,9 @@ brm_summary(
   time = "AVISIT",
   patient = "USUBJID",
   covariates = character(0),
-  control = "Placebo"
+  control = "Placebo",
+  baseline = "Baseline",
+  response_type = "change"
 )
 }
 \arguments{
@@ -32,8 +34,17 @@ in the data.}
 \item{covariates}{Character vector of names of other covariates
 in the data.}
 
-\item{control}{Character of length 1, name of the control arm
-in the \code{group} column in the data.}
+\item{control}{Element of the \code{group} column in the data which indicates
+the control group for the purposes of calculating treatment differences.}
+
+\item{baseline}{Element of the \code{time} column in the data
+which indicates the baseline time for the purposes of calculating
+change from baseline. Ignored if \code{response_type = "change"}.}
+
+\item{response_type}{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).}
 }
 \value{
 A \code{tibble} with summary statistics of the marginal posterior.
@@ -49,6 +60,8 @@ Currently assumes the response variable is \code{CHG}
 set.seed(0L)
 sim <- brm_simulate()
 data <- sim$data
+data$group <- paste("treatment", data$group)
+data$time <- paste("visit", data$time)
 formula <- brm_formula(
   response = "response",
   group = "group",
@@ -75,7 +88,9 @@ brm_summary(
   group = "group",
   time = "time",
   patient = "patient",
-  control = 1
+  control = "treatment 1",
+  baseline = "visit 1",
+  response_type = "response"
 )
 }
 \concept{results}