diff --git a/R/meta_boxly.R b/R/meta_boxly.R
index 5a74ea1..09e8201 100644
--- a/R/meta_boxly.R
+++ b/R/meta_boxly.R
@@ -41,13 +41,14 @@
 #'   observation_term = "wk12"
 #' )
 meta_boxly <- function(
-    dataset_adsl,
-    dataset_param,
-    population_term,
-    population_subset = SAFFL == "Y",
-    observation_term,
-    observation_subset = SAFFL == "Y",
-    parameters = unique(dataset_param$PARAMCD)) {
+  dataset_adsl,
+  dataset_param,
+  population_term,
+  population_subset = SAFFL == "Y",
+  observation_term,
+  observation_subset = SAFFL == "Y",
+  parameters = unique(dataset_param$PARAMCD)
+) {
   # Input Checking
   require_param <- c("PARAM", "PARAMCD", "AVISITN", "CHG")
 
diff --git a/R/prepare_boxly.R b/R/prepare_boxly.R
index 1cf3e53..483189a 100644
--- a/R/prepare_boxly.R
+++ b/R/prepare_boxly.R
@@ -25,6 +25,8 @@
 #'   The term name is used as key to link information.
 #' @param analysis A character value of analysis term name.
 #'   The term name is used as key to link information.
+#' @param filter_var A character value of variable name used for filtering.
+#'   Default is "PARAM".
 #' @param hover_var_outlier A character vector of hover variables for outlier.
 #'
 #' @return Metadata list with plotting dataset.
@@ -48,6 +50,7 @@ prepare_boxly <- function(meta,
                           population = NULL,
                           observation = NULL,
                           analysis = NULL,
+                          filter_var = "PARAM",
                           hover_var_outlier = c("USUBJID", metalite::collect_adam_mapping(meta, analysis)$y)) {
   if (is.null(population)) {
     if (length(meta$population) == 1) {
@@ -109,11 +112,23 @@ prepare_boxly <- function(meta,
     obs[[obs_group]] <- factor(obs[[obs_group]], levels = sort(unique(obs[[obs_group]])))
   }
 
-  if (!"factor" %in% class(obs[[obs_var]])) {
-    message("In observation level data, the facet variable '", obs_var, "' is automatically transformed into a factor.")
-    obs[[obs_var]] <- factor(obs[[obs_var]], levels = sort(unique(obs[[obs_var]])))
+  if (!filter_var %in% obs_var) {
+    stop(paste(
+      "The filter variable '", filter_var, "' is not found in the observation data.",
+      "Please check the metadata for observation and `filter_var`."
+    ))
   }
 
+  obs[, obs_var] <- lapply(obs_var, function(var) {
+    x <- obs[[var]]
+    if (!is.factor(x)) {
+      message("In observation level data, the facet variable '", var, "' is automatically transformed into a factor.")
+      factor(x, levels = sort(unique(x)))
+    } else {
+      x
+    }
+  })
+
   if (!"factor" %in% class(obs[[x]])) {
     message("In observation level data, the group variable '", x, "' is automatically transformed into a factor.")
     obs[[x]] <- factor(obs[[x]], levels = sort(unique(obs[[x]])))
@@ -163,8 +178,8 @@ prepare_boxly <- function(meta,
         ans
       }
     },
-    split(tbl, list(tbl[[obs_var]], tbl[[obs_group]], tbl[[x]])),
-    names(split(tbl, list(tbl[[obs_var]], tbl[[obs_group]], tbl[[x]]), sep = ", ")),
+    split(tbl, tbl[, c(obs_var, obs_group, x)]),
+    names(split(tbl, tbl[, c(obs_var, obs_group, x)], sep = ", ")),
     SIMPLIFY = FALSE
   )
 
@@ -198,7 +213,7 @@ prepare_boxly <- function(meta,
   # Return value
   metalite::outdata(meta, population, observation, parameter,
     x_var = x, y_var = y, group_var = obs_group,
-    param_var = obs_var, hover_var_outlier = hover_var_outlier,
+    param_var = filter_var, hover_var_outlier = hover_var_outlier,
     n = n_tbl, order = NULL, group = NULL, reference_group = NULL,
     plotds = plotds
   )
diff --git a/man/prepare_boxly.Rd b/man/prepare_boxly.Rd
index d917fe3..8318bb6 100644
--- a/man/prepare_boxly.Rd
+++ b/man/prepare_boxly.Rd
@@ -9,6 +9,7 @@ prepare_boxly(
   population = NULL,
   observation = NULL,
   analysis = NULL,
+  filter_var = "PARAM",
   hover_var_outlier = c("USUBJID", metalite::collect_adam_mapping(meta, analysis)$y)
 )
 }
@@ -24,6 +25,9 @@ The term name is used as key to link information.}
 \item{analysis}{A character value of analysis term name.
 The term name is used as key to link information.}
 
+\item{filter_var}{A character value of variable name used for filtering.
+Default is "PARAM".}
+
 \item{hover_var_outlier}{A character vector of hover variables for outlier.}
 }
 \value{