diff --git a/NAMESPACE b/NAMESPACE
index 5129b999..a7b53597 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -34,6 +34,7 @@ export(ard_effectsize_hedges_g)
 export(ard_effectsize_paired_cohens_d)
 export(ard_effectsize_paired_hedges_g)
 export(ard_emmeans_mean_difference)
+export(ard_event_rates)
 export(ard_missing)
 export(ard_regression)
 export(ard_regression_basic)
diff --git a/R/ard_event_rates.R b/R/ard_event_rates.R
new file mode 100644
index 00000000..73e72142
--- /dev/null
+++ b/R/ard_event_rates.R
@@ -0,0 +1,173 @@
+#' ARD to Calculate Event Occurrence Rates by ID
+#'
+#' Function calculates event occurrences rates per unique ID.
+#' Each variable in `variables` is evaluated independently and then results for all variables are stacked.
+#' For non-ordered variables (`ordered = FALSE`), each level that occurs per unique ID will be counted once.
+#' For ordered variables (`ordered = TRUE`), only the highest-ordered level will be counted for each unique ID.
+#'
+#' @inheritParams cards::ard_categorical
+#' @inheritParams cards::ard_stack
+#' @param variables ([`tidy-select`][dplyr::dplyr_tidy_select])\cr
+#'   The factor variables for which event rates (for each level) will be calculated.
+#' @param id ([`tidy-select`][dplyr::dplyr_tidy_select])\cr
+#'   Argument used to subset `data` to identify rows in `data` to calculate event rates.
+#' @param denominator (`data.frame`, `integer`)\cr
+#'   Used to define the denominator and enhance the output.
+#'   The argument is optional. If not specified, `data` will be used as `denominator`.
+#'   - the univariate tabulations of the `by` variables are calculated with `denominator` when a data frame is passed,
+#'     e.g. tabulation of the treatment assignment counts that may appear in the header of a table.
+#' @param ordered (`logical`)\cr
+#'   Specifies whether factor variables specified by `variables` are ordered or not. If ordered, only the
+#'   highest-ordered level will be counted for each unique value of `id`. Otherwise, each level that occurs per unique
+#'   value of `id` will be counted once. Must be the same length as `variables`. Defaults to `TRUE` for ordered factor
+#'   variables and `FALSE` otherwise.
+#'
+#' @return an ARD data frame of class 'card'
+#' @name ard_event_rates
+#'
+#' @examples
+#' # Example 1 - Event Rates ------------------------------------
+#' ard_event_rates(
+#'   cards::ADAE,
+#'   variables = c(AEBODSYS, AESOC),
+#'   id = USUBJID,
+#'   by = TRTA,
+#'   denominator = cards::ADSL |> dplyr::rename(TRTA = ARM)
+#' )
+#'
+#' # Example 2 - Event Rates by Highest Severity ----------------
+#' ard_event_rates(
+#'   cards::ADAE,
+#'   variables = AESEV,
+#'   id = USUBJID,
+#'   by = TRTA,
+#'   denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+#'   ordered = TRUE
+#' )
+NULL
+
+#' @rdname ard_event_rates
+#' @export
+ard_event_rates <- function(data,
+                            variables,
+                            id,
+                            by = dplyr::group_vars(data),
+                            statistic = everything() ~ c("n", "p", "N"),
+                            denominator = NULL,
+                            fmt_fn = NULL,
+                            stat_label = everything() ~ cards::default_stat_labels(),
+                            ordered = sapply(data[variables], is.ordered),
+                            ...) {
+  set_cli_abort_call()
+
+  # check inputs ---------------------------------------------------------------
+  check_not_missing(data)
+  check_not_missing(variables)
+  check_not_missing(id)
+  cards::process_selectors(data, variables = {{ variables }}, id = {{ id }}, by = {{ by }})
+  data <- dplyr::ungroup(data)
+
+  # denominator must a data frame, or integer
+  if (!is_empty(denominator) && !is.data.frame(denominator) && !is_integerish(denominator)) {
+    cli::cli_abort(
+      "The {.arg denominator} argument must be a {.cls data.frame} or an {.cls integer}, not {.obj_type_friendly {denominator}}.",
+      call = get_cli_abort_call()
+    )
+  }
+  if (is_empty(denominator)) denominator <- data
+
+  # check the id argument is not empty
+  if (is_empty(id)) {
+    cli::cli_abort("Argument {.arg id} cannot be empty.", call = get_cli_abort_call())
+  }
+
+  # return empty ARD if no variables selected ----------------------------------
+  if (is_empty(variables)) {
+    return(dplyr::tibble() |> cards::as_card())
+  }
+
+  # check the ordered argument
+  check_logical(ordered)
+  if (length(ordered) != length(variables)) {
+    cli::cli_abort(
+      "Argument {.arg ordered} has length {length(ordered)} but must be the same length as {.arg variables} ({length(variables)}).",
+      call = get_cli_abort_call()
+    )
+  }
+
+  # drop missing values --------------------------------------------------------
+  df_na_nan <- is.na(data[c(by, variables)]) | apply(data[c(by, variables)], MARGIN = 2, is.nan)
+  if (any(df_na_nan)) {
+    rows_with_na <- apply(df_na_nan, MARGIN = 1, any)
+    cli::cli_inform(c("*" = "Removing {.val {sum(rows_with_na)}} row{?s} from {.arg data} with
+                            {.val {NA}} or {.val {NaN}} values in {.val {c(by, variables)}} column{?s}."))
+    data <- data[!rows_with_na, ]
+  }
+
+  # remove missing by variables from `denominator`
+  if (is.data.frame(denominator) && !is_empty(intersect(by, names(denominator)))) {
+    df_na_nan_denom <-
+      is.na(denominator[intersect(by, names(denominator))]) |
+        apply(denominator[intersect(by, names(denominator))], MARGIN = 2, is.nan)
+    if (any(df_na_nan_denom)) {
+      rows_with_na_denom <- apply(df_na_nan_denom, MARGIN = 1, any)
+      cli::cli_inform(c("*" = "Removing {.val {sum(rows_with_na_denom)}} row{?s} from {.arg denominator} with
+                              {.val {NA}} or {.val {NaN}} values in {.val {intersect(by, names(denominator))}} column{?s}."))
+      denominator <- denominator[!rows_with_na_denom, ]
+    }
+  }
+
+  # sort data ------------------------------------------------------------------
+  data <- dplyr::arrange(data, dplyr::pick(all_of(c(id, by, variables))))
+
+  # print denom columns if not 100% clear which are used
+  if (!is_empty(id) && is.data.frame(denominator)) {
+    denom_cols <- intersect(by, names(denominator))
+    if (!setequal(by, denom_cols)) {
+      msg <-
+        ifelse(
+          is_empty(denom_cols),
+          "Denominator set by number of rows in {.arg denominator} data frame.",
+          "Denominator set by {.val {denom_cols}} column{?s} in {.arg denominator} data frame."
+        )
+      cli::cli_inform(c("i" = msg))
+    }
+  }
+
+  lst_results <- list()
+  for (var in variables) {
+    ord <- (is_named(ordered) && ordered[var]) || (!is_named(ordered) && ordered[which(variables == var)])
+    if (ord) data[[var]] <- factor(data[[var]], ordered = TRUE)
+
+    lst_results <-
+      lst_results |>
+      append(
+        ard_categorical(
+          data = data |>
+            dplyr::slice_tail(n = 1L, by = all_of(c(id, intersect(by, names(denominator)), if (!ord) var))),
+          variables = all_of(var),
+          by = all_of(by),
+          statistic = statistic,
+          denominator = denominator,
+          fmt_fn = fmt_fn,
+          stat_label = stat_label
+        ) |>
+          list()
+      )
+
+    if (ord) {
+      lst_results[[length(lst_results)]] <- lst_results[[length(lst_results)]] |>
+        dplyr::mutate(variable_level = as.list(as.character(unlist(.data$variable_level))))
+    }
+  }
+
+  # combine results ------------------------------------------------------------
+  result <- lst_results |>
+    dplyr::bind_rows() |>
+    dplyr::mutate(context = "event_rates") |>
+    cards::tidy_ard_column_order() |>
+    cards::tidy_ard_row_order()
+
+  # return final result --------------------------------------------------------
+  result
+}
diff --git a/_pkgdown.yml b/_pkgdown.yml
index c129d233..e1cfcec2 100644
--- a/_pkgdown.yml
+++ b/_pkgdown.yml
@@ -88,6 +88,7 @@ reference:
       - ard_categorical_ci.data.frame
       - ard_regression
       - ard_regression_basic
+      - ard_event_rates
 
   - title: "Helpers"
   - contents:
diff --git a/man/ard_event_rates.Rd b/man/ard_event_rates.Rd
new file mode 100644
index 00000000..ba3a3a6f
--- /dev/null
+++ b/man/ard_event_rates.Rd
@@ -0,0 +1,95 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/ard_event_rates.R
+\name{ard_event_rates}
+\alias{ard_event_rates}
+\title{ARD to Calculate Event Occurrence Rates by ID}
+\usage{
+ard_event_rates(
+  data,
+  variables,
+  id,
+  by = dplyr::group_vars(data),
+  statistic = everything() ~ c("n", "p", "N"),
+  denominator = NULL,
+  fmt_fn = NULL,
+  stat_label = everything() ~ cards::default_stat_labels(),
+  ordered = sapply(data[variables], is.ordered),
+  ...
+)
+}
+\arguments{
+\item{data}{(\code{data.frame})\cr
+a data frame}
+
+\item{variables}{(\code{\link[dplyr:dplyr_tidy_select]{tidy-select}})\cr
+The factor variables for which event rates (for each level) will be calculated.}
+
+\item{id}{(\code{\link[dplyr:dplyr_tidy_select]{tidy-select}})\cr
+Argument used to subset \code{data} to identify rows in \code{data} to calculate event rates.}
+
+\item{by}{(\code{\link[dplyr:dplyr_tidy_select]{tidy-select}})\cr
+columns to tabulate by in the series of ARD function calls.
+Any rows with \code{NA} or \code{NaN} values are removed from all calculations.}
+
+\item{statistic}{(\code{\link[cards:syntax]{formula-list-selector}})\cr
+a named list, a list of formulas,
+or a single formula where the list element one or more of  \code{c("n", "N", "p")}
+(or the RHS of a formula).}
+
+\item{denominator}{(\code{data.frame}, \code{integer})\cr
+Used to define the denominator and enhance the output.
+The argument is optional. If not specified, \code{data} will be used as \code{denominator}.
+\itemize{
+\item the univariate tabulations of the \code{by} variables are calculated with \code{denominator} when a data frame is passed,
+e.g. tabulation of the treatment assignment counts that may appear in the header of a table.
+}}
+
+\item{fmt_fn}{(\code{\link[cards:syntax]{formula-list-selector}})\cr
+a named list, a list of formulas,
+or a single formula where the list element is a named list of functions
+(or the RHS of a formula),
+e.g. \verb{list(mpg = list(mean = \\(x) round(x, digits = 2) |> as.character()))}.}
+
+\item{stat_label}{(\code{\link[cards:syntax]{formula-list-selector}})\cr
+a named list, a list of formulas, or a single formula where
+the list element is either a named list or a list of formulas defining the
+statistic labels, e.g. \code{everything() ~ list(n = "n", p = "pct")} or
+\code{everything() ~ list(n ~ "n", p ~ "pct")}.}
+
+\item{ordered}{(\code{logical})\cr
+Specifies whether factor variables specified by \code{variables} are ordered or not. If ordered, only the
+highest-ordered level will be counted for each unique value of \code{id}. Otherwise, each level that occurs per unique
+value of \code{id} will be counted once. Must be the same length as \code{variables}. Defaults to \code{TRUE} for ordered factor
+variables and \code{FALSE} otherwise.}
+
+\item{...}{Arguments passed to methods.}
+}
+\value{
+an ARD data frame of class 'card'
+}
+\description{
+Function calculates event occurrences rates per unique ID.
+Each variable in \code{variables} is evaluated independently and then results for all variables are stacked.
+For non-ordered variables (\code{ordered = FALSE}), each level that occurs per unique ID will be counted once.
+For ordered variables (\code{ordered = TRUE}), only the highest-ordered level will be counted for each unique ID.
+}
+\examples{
+# Example 1 - Event Rates ------------------------------------
+ard_event_rates(
+  cards::ADAE,
+  variables = c(AEBODSYS, AESOC),
+  id = USUBJID,
+  by = TRTA,
+  denominator = cards::ADSL |> dplyr::rename(TRTA = ARM)
+)
+
+# Example 2 - Event Rates by Highest Severity ----------------
+ard_event_rates(
+  cards::ADAE,
+  variables = AESEV,
+  id = USUBJID,
+  by = TRTA,
+  denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+  ordered = TRUE
+)
+}
diff --git a/tests/testthat/_snaps/ard_event_rates.md b/tests/testthat/_snaps/ard_event_rates.md
new file mode 100644
index 00000000..20f7c55b
--- /dev/null
+++ b/tests/testthat/_snaps/ard_event_rates.md
@@ -0,0 +1,151 @@
+# ard_event_rates() works with default settings
+
+    Code
+      print(res, n = 20, columns = "all")
+    Message
+      {cards} data frame: 207 x 11
+    Output
+         group1 group1_level variable variable_level   context stat_name stat_label  stat fmt_fn warning error
+      1    TRTA      Placebo    AESOC      CARDIAC … event_ra…         n          n    13      0              
+      2    TRTA      Placebo    AESOC      CARDIAC … event_ra…         N          N   301      0              
+      3    TRTA      Placebo    AESOC      CARDIAC … event_ra…         p          % 0.043   <fn>              
+      4    TRTA      Placebo    AESOC      CONGENIT… event_ra…         n          n     0      0              
+      5    TRTA      Placebo    AESOC      CONGENIT… event_ra…         N          N   301      0              
+      6    TRTA      Placebo    AESOC      CONGENIT… event_ra…         p          %     0   <fn>              
+      7    TRTA      Placebo    AESOC      EAR AND … event_ra…         n          n     1      0              
+      8    TRTA      Placebo    AESOC      EAR AND … event_ra…         N          N   301      0              
+      9    TRTA      Placebo    AESOC      EAR AND … event_ra…         p          % 0.003   <fn>              
+      10   TRTA      Placebo    AESOC      EYE DISO… event_ra…         n          n     4      0              
+      11   TRTA      Placebo    AESOC      EYE DISO… event_ra…         N          N   301      0              
+      12   TRTA      Placebo    AESOC      EYE DISO… event_ra…         p          % 0.013   <fn>              
+      13   TRTA      Placebo    AESOC      GASTROIN… event_ra…         n          n    17      0              
+      14   TRTA      Placebo    AESOC      GASTROIN… event_ra…         N          N   301      0              
+      15   TRTA      Placebo    AESOC      GASTROIN… event_ra…         p          % 0.056   <fn>              
+      16   TRTA      Placebo    AESOC      GENERAL … event_ra…         n          n    21      0              
+      17   TRTA      Placebo    AESOC      GENERAL … event_ra…         N          N   301      0              
+      18   TRTA      Placebo    AESOC      GENERAL … event_ra…         p          %  0.07   <fn>              
+      19   TRTA      Placebo    AESOC      HEPATOBI… event_ra…         n          n     1      0              
+      20   TRTA      Placebo    AESOC      HEPATOBI… event_ra…         N          N   301      0              
+    Message
+      i 187 more rows
+      i Use `print(n = ...)` to see more rows
+
+---
+
+    Code
+      print(ard_event_rates(dplyr::group_by(cards::ADAE, TRTA), variables = AESOC, id = USUBJID, denominator = dplyr::rename(cards::ADSL, TRTA = ARM)), n = 20, columns = "all")
+    Message
+      {cards} data frame: 207 x 11
+    Output
+         group1 group1_level variable variable_level   context stat_name stat_label  stat fmt_fn warning error
+      1    TRTA      Placebo    AESOC      CARDIAC … event_ra…         n          n    13      0              
+      2    TRTA      Placebo    AESOC      CARDIAC … event_ra…         N          N    86      0              
+      3    TRTA      Placebo    AESOC      CARDIAC … event_ra…         p          % 0.151   <fn>              
+      4    TRTA      Placebo    AESOC      CONGENIT… event_ra…         n          n     0      0              
+      5    TRTA      Placebo    AESOC      CONGENIT… event_ra…         N          N    86      0              
+      6    TRTA      Placebo    AESOC      CONGENIT… event_ra…         p          %     0   <fn>              
+      7    TRTA      Placebo    AESOC      EAR AND … event_ra…         n          n     1      0              
+      8    TRTA      Placebo    AESOC      EAR AND … event_ra…         N          N    86      0              
+      9    TRTA      Placebo    AESOC      EAR AND … event_ra…         p          % 0.012   <fn>              
+      10   TRTA      Placebo    AESOC      EYE DISO… event_ra…         n          n     4      0              
+      11   TRTA      Placebo    AESOC      EYE DISO… event_ra…         N          N    86      0              
+      12   TRTA      Placebo    AESOC      EYE DISO… event_ra…         p          % 0.047   <fn>              
+      13   TRTA      Placebo    AESOC      GASTROIN… event_ra…         n          n    17      0              
+      14   TRTA      Placebo    AESOC      GASTROIN… event_ra…         N          N    86      0              
+      15   TRTA      Placebo    AESOC      GASTROIN… event_ra…         p          % 0.198   <fn>              
+      16   TRTA      Placebo    AESOC      GENERAL … event_ra…         n          n    21      0              
+      17   TRTA      Placebo    AESOC      GENERAL … event_ra…         N          N    86      0              
+      18   TRTA      Placebo    AESOC      GENERAL … event_ra…         p          % 0.244   <fn>              
+      19   TRTA      Placebo    AESOC      HEPATOBI… event_ra…         n          n     1      0              
+      20   TRTA      Placebo    AESOC      HEPATOBI… event_ra…         N          N    86      0              
+    Message
+      i 187 more rows
+      i Use `print(n = ...)` to see more rows
+
+# ard_event_rates(statistic) works
+
+    Code
+      ard_event_rates(cards::ADAE, variables = SEX, id = USUBJID, by = TRTA, denominator = dplyr::rename(cards::ADSL, TRTA = ARM), statistic = ~"n")
+    Message
+      {cards} data frame: 6 x 11
+    Output
+        group1 group1_level variable variable_level stat_name stat_label stat
+      1   TRTA      Placebo      SEX              F         n          n   40
+      2   TRTA      Placebo      SEX              M         n          n   29
+      3   TRTA    Xanomeli…      SEX              F         n          n   37
+      4   TRTA    Xanomeli…      SEX              M         n          n   42
+      5   TRTA    Xanomeli…      SEX              F         n          n   44
+      6   TRTA    Xanomeli…      SEX              M         n          n   33
+    Message
+      i 4 more variables: context, fmt_fn, warning, error
+
+# ard_event_rates(ordered) works
+
+    Code
+      print(res, n = 20, columns = "all")
+    Message
+      {cards} data frame: 27 x 11
+    Output
+         group1 group1_level variable variable_level   context stat_name stat_label  stat fmt_fn warning error
+      1    TRTA      Placebo    AESEV           MILD event_ra…         n          n    36      0              
+      2    TRTA      Placebo    AESEV           MILD event_ra…         N          N    86      0              
+      3    TRTA      Placebo    AESEV           MILD event_ra…         p          % 0.419   <fn>              
+      4    TRTA      Placebo    AESEV       MODERATE event_ra…         n          n    26      0              
+      5    TRTA      Placebo    AESEV       MODERATE event_ra…         N          N    86      0              
+      6    TRTA      Placebo    AESEV       MODERATE event_ra…         p          % 0.302   <fn>              
+      7    TRTA      Placebo    AESEV         SEVERE event_ra…         n          n     7      0              
+      8    TRTA      Placebo    AESEV         SEVERE event_ra…         N          N    86      0              
+      9    TRTA      Placebo    AESEV         SEVERE event_ra…         p          % 0.081   <fn>              
+      10   TRTA    Xanomeli…    AESEV           MILD event_ra…         n          n    22      0              
+      11   TRTA    Xanomeli…    AESEV           MILD event_ra…         N          N    84      0              
+      12   TRTA    Xanomeli…    AESEV           MILD event_ra…         p          % 0.262   <fn>              
+      13   TRTA    Xanomeli…    AESEV       MODERATE event_ra…         n          n    49      0              
+      14   TRTA    Xanomeli…    AESEV       MODERATE event_ra…         N          N    84      0              
+      15   TRTA    Xanomeli…    AESEV       MODERATE event_ra…         p          % 0.583   <fn>              
+      16   TRTA    Xanomeli…    AESEV         SEVERE event_ra…         n          n     8      0              
+      17   TRTA    Xanomeli…    AESEV         SEVERE event_ra…         N          N    84      0              
+      18   TRTA    Xanomeli…    AESEV         SEVERE event_ra…         p          % 0.095   <fn>              
+      19   TRTA    Xanomeli…    AESEV           MILD event_ra…         n          n    19      0              
+      20   TRTA    Xanomeli…    AESEV           MILD event_ra…         N          N    84      0              
+    Message
+      i 7 more rows
+      i Use `print(n = ...)` to see more rows
+
+---
+
+    Code
+      ard_event_rates(adae, variables = c(SEX, AESEV), id = USUBJID, by = TRTA, denominator = dplyr::rename(cards::ADSL, TRTA = ARM), ordered = TRUE)
+    Condition
+      Error in `ard_event_rates()`:
+      ! Argument `ordered` has length 1 but must be the same length as `variables` (2).
+
+# ard_event_rates() errors with incomplete factor columns
+
+    Code
+      ard_event_rates(dplyr::mutate(cards::ADAE, AESOC = factor(AESOC, levels = character(
+        0))), variables = AESOC, id = USUBJID, by = TRTA)
+    Message
+      * Removing 1191 rows from `data` with NA or NaN values in "TRTA" and "AESOC" columns.
+    Condition
+      Error in `ard_event_rates()`:
+      ! Factors with empty "levels" attribute are not allowed, which was identified in column "AESOC".
+
+---
+
+    Code
+      ard_event_rates(dplyr::mutate(cards::ADAE, SEX = factor(SEX, levels = c("F",
+        "M", NA), exclude = NULL)), variables = SEX, id = USUBJID, by = TRTA)
+    Condition
+      Error in `ard_event_rates()`:
+      ! Factors with NA levels are not allowed, which are present in column "SEX".
+
+# ard_event_rates() works without any variables
+
+    Code
+      ard_event_rates(data = cards::ADAE, variables = starts_with("xxxx"), id = USUBJID,
+      by = c(TRTA, AESEV))
+    Message
+      {cards} data frame: 0 x 0
+    Output
+      data frame with 0 columns and 0 rows
+
diff --git a/tests/testthat/test-ard_event_rates.R b/tests/testthat/test-ard_event_rates.R
new file mode 100644
index 00000000..652979ba
--- /dev/null
+++ b/tests/testthat/test-ard_event_rates.R
@@ -0,0 +1,251 @@
+test_that("ard_event_rates() works with default settings", {
+  withr::local_options(list(width = 200))
+
+  expect_silent(
+    res <- ard_event_rates(
+      cards::ADAE,
+      variables = AESOC,
+      id = USUBJID,
+      by = TRTA
+    )
+  )
+  expect_snapshot(res |> print(n = 20, columns = "all"))
+
+  expect_equal(
+    res |>
+      dplyr::filter(
+        group1_level == "Placebo",
+        variable_level == "CARDIAC DISORDERS",
+        stat_name == "n"
+      ) |>
+      cards::get_ard_statistics(),
+    list(
+      n = cards::ADAE |>
+        dplyr::filter(
+          TRTA == "Placebo",
+          AESOC == "CARDIAC DISORDERS"
+        ) |>
+        dplyr::slice_tail(n = 1L, by = all_of(c("USUBJID", "TRTA", "AESOC"))) |>
+        nrow()
+    )
+  )
+
+  # with denominator
+  expect_snapshot(
+    ard_event_rates(
+      cards::ADAE |> dplyr::group_by(TRTA),
+      variables = AESOC,
+      id = USUBJID,
+      denominator = cards::ADSL |> dplyr::rename(TRTA = ARM)
+    ) |>
+      print(n = 20, columns = "all")
+  )
+
+  # with multiple variables
+  expect_silent(
+    res2 <- ard_event_rates(
+      cards::ADAE,
+      variables = c(SEX, AESOC),
+      id = USUBJID,
+      by = TRTA
+    )
+  )
+  expect_equal(unique(res2$variable), c("SEX", "AESOC"))
+  expect_equal(
+    res,
+    res2[-c(1:18), ]
+  )
+})
+
+test_that("ard_event_rates(statistic) works", {
+  withr::local_options(list(width = 200))
+
+  expect_snapshot(
+    ard_event_rates(
+      cards::ADAE,
+      variables = SEX,
+      id = USUBJID,
+      by = TRTA,
+      denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+      statistic = ~"n"
+    )
+  )
+})
+
+test_that("ard_event_rates(ordered) works", {
+  withr::local_options(list(width = 200))
+
+  # pre-ordered factor variable
+  adae <- cards::ADAE |>
+    dplyr::mutate(AESEV = factor(cards::ADAE$AESEV, ordered = TRUE))
+
+  expect_silent(
+    res <- ard_event_rates(
+      cards::ADAE,
+      variables = AESEV,
+      id = USUBJID,
+      by = TRTA,
+      denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+      ordered = TRUE
+    )
+  )
+  expect_snapshot(res |> print(n = 20, columns = "all"))
+
+  expect_equal(
+    res |>
+      dplyr::filter(
+        group1_level == "Placebo",
+        variable_level == "MODERATE",
+        stat_name == "n"
+      ) |>
+      cards::get_ard_statistics(),
+    list(
+      n = adae |>
+        dplyr::arrange(AESEV) |>
+        dplyr::slice_tail(n = 1L, by = all_of(c("USUBJID", "TRTA"))) |>
+        dplyr::filter(
+          TRTA == "Placebo",
+          AESEV == "MODERATE"
+        ) |>
+        nrow()
+    )
+  )
+
+  res_unord <- ard_event_rates(
+    cards::ADAE,
+    variables = AESEV,
+    id = USUBJID,
+    by = TRTA,
+    denominator = cards::ADSL |> dplyr::rename(TRTA = ARM)
+  )
+  expect_true(res$stat[[1]] != res_unord$stat[[1]])
+
+  res2 <- ard_event_rates(
+    adae,
+    variables = AESEV,
+    id = USUBJID,
+    by = TRTA,
+    denominator = cards::ADSL |> dplyr::rename(TRTA = ARM)
+  )
+  expect_equal(res, res2)
+
+  # multiple variables
+  expect_silent(
+    res3 <- ard_event_rates(
+      adae,
+      variables = c(SEX, AESEV),
+      id = USUBJID,
+      by = TRTA,
+      denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+      ordered = c(FALSE, TRUE)
+    )
+  )
+  expect_equal(res, res3[-c(1:18), ])
+
+  # named vector
+  expect_silent(
+    res4 <- ard_event_rates(
+      adae,
+      variables = c(SEX, AESEV),
+      id = USUBJID,
+      by = TRTA,
+      denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+      ordered = c(AESEV = TRUE, SEX = FALSE)
+    )
+  )
+  expect_equal(res3, res4)
+
+  # error - length does not match
+  expect_snapshot(
+    ard_event_rates(
+      adae,
+      variables = c(SEX, AESEV),
+      id = USUBJID,
+      by = TRTA,
+      denominator = cards::ADSL |> dplyr::rename(TRTA = ARM),
+      ordered = TRUE
+    ),
+    error = TRUE
+  )
+  expect_equal(res, res2)
+})
+
+test_that("ard_event_rates() errors with incomplete factor columns", {
+  # Check error when factors have no levels
+  expect_snapshot(
+    error = TRUE,
+    ard_event_rates(
+      cards::ADAE |>
+        dplyr::mutate(AESOC = factor(AESOC, levels = character(0))),
+      variables = AESOC,
+      id = USUBJID,
+      by = TRTA
+    )
+  )
+
+  # Check error when factor has NA level
+  expect_snapshot(
+    error = TRUE,
+    ard_event_rates(
+      cards::ADAE |>
+        dplyr::mutate(SEX = factor(SEX, levels = c("F", "M", NA), exclude = NULL)),
+      variables = SEX,
+      id = USUBJID,
+      by = TRTA
+    )
+  )
+})
+
+test_that("ard_hierarchical_count() works with by variable not present in 'denominator'", {
+  expect_silent(
+    ard_events_with_by <- ard_event_rates(
+      data = cards::ADAE,
+      variables = AESOC,
+      id = USUBJID,
+      by = c(TRTA, AESEV),
+      statistic = ~"n"
+    )
+  )
+
+  expect_equal(
+    ard_events_with_by |>
+      dplyr::filter(
+        group1_level == "Placebo",
+        group2_level == "MILD",
+        variable_level == "CARDIAC DISORDERS"
+      ) |>
+      cards::get_ard_statistics(),
+    list(
+      n = cards::ADAE |>
+        dplyr::filter(
+          TRTA == "Placebo",
+          AESEV == "MILD",
+          AESOC == "CARDIAC DISORDERS"
+        ) |>
+        dplyr::slice_tail(n = 1L, by = all_of(c("USUBJID", "TRTA", "AESEV", "AESOC"))) |>
+        nrow()
+    )
+  )
+})
+
+test_that("ard_event_rates() works without any variables", {
+  expect_snapshot(
+    ard_event_rates(
+      data = cards::ADAE,
+      variables = starts_with("xxxx"),
+      id = USUBJID,
+      by = c(TRTA, AESEV)
+    )
+  )
+})
+
+test_that("ard_event_rates() follows ard structure", {
+  expect_silent(
+    ard_event_rates(
+      cards::ADAE,
+      variables = AESOC,
+      id = USUBJID
+    ) |>
+      cards::check_ard_structure(method = FALSE)
+  )
+})