diff --git a/DESCRIPTION b/DESCRIPTION
index 97577506..9d3ce499 100755
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -51,6 +51,8 @@ Imports:
   tidyr,
   tsibble
 Suggests:
+    testthat (>= 3.0.0),
     delphi.epidata
 Remotes:
     github::cmu-delphi/delphi-epidata-r
+Config/testthat/edition: 3
diff --git a/NAMESPACE b/NAMESPACE
index b077342e..78363e2b 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -64,4 +64,5 @@ importFrom(stats,cor)
 importFrom(stats,median)
 importFrom(tidyr,unnest)
 importFrom(tidyselect,eval_select)
+importFrom(tidyselect,starts_with)
 importFrom(tsibble,as_tsibble)
diff --git a/R/outliers.R b/R/outliers.R
index f12b99e5..9bd370dc 100644
--- a/R/outliers.R
+++ b/R/outliers.R
@@ -45,12 +45,15 @@
 #' 
 #' @export
 detect_outlr = function(x = seq_along(y), y,
-                        methods = tibble(method = "rm",
+                        methods = tibble::tibble(method = "rm",
                                          args = list(list()),
                                          abbr = "rm"),
                         combiner = c("median", "mean", "none")) {
   # Validate combiner
   combiner = match.arg(combiner)
+  
+  # Validate that x contains all distinct values
+  if (max(table(x)) > 1) Abort("`x` must not contain duplicate values; did you group your `epi_df` by all relevant key variables?")
 
   # Run all outlier detection methods
   results = purrr::pmap_dfc(methods, function(method, args, abbr) {
@@ -187,6 +190,7 @@ detect_outlr_rm = function(x = seq_along(y), y, n = 21,
 #'   description.
 #'
 #' @importFrom stats median
+#' @importFrom tidyselect starts_with
 #' @export
 detect_outlr_stl = function(x = seq_along(y), y,
                             n_trend = 21,
@@ -216,11 +220,10 @@ detect_outlr_stl = function(x = seq_along(y), y,
     fabletools::model(feasts::STL(stl_formula, robust = TRUE)) %>%
     generics::components() %>%
     tibble::as_tibble() %>%
-    dplyr::transmute(
-      trend = trend,
-      seasonal = season_week,
-      resid = remainder)
-  
+    dplyr::select(trend:remainder) %>%
+    dplyr::rename_with(~ "seasonal", tidyselect::starts_with("season")) %>%
+    dplyr::rename(resid = remainder)
+    
   # Allocate the seasonal term from STL to either fitted or resid 
   if (!is.null(seasonal_period)) {
     stl_components = stl_components %>%
@@ -263,15 +266,18 @@ detect_outlr_stl = function(x = seq_along(y), y,
 
 # Common function for rolling IQR, using fitted and resid variables
 roll_iqr = function(z, n, detection_multiplier, min_radius,
-                    replacement_multiplier, min_lower) { 
+                    replacement_multiplier, min_lower) {
+  if (typeof(z$y) == "integer") as_type = as.integer
+  else as_type = as.numeric
+  
   epi_slide(z, roll_iqr = IQR(resid), n = n, align = "center") %>%
     dplyr::mutate(
       lower = pmax(min_lower,
                    fitted - pmax(min_radius, detection_multiplier * roll_iqr)),
       upper = fitted + pmax(min_radius, detection_multiplier * roll_iqr),
       replacement = dplyr::case_when(
-      (y < lower) ~ fitted - replacement_multiplier * roll_iqr,
-      (y > upper) ~ fitted + replacement_multiplier * roll_iqr,
+      (y < lower) ~ as_type(fitted - replacement_multiplier * roll_iqr),
+      (y > upper) ~ as_type(fitted + replacement_multiplier * roll_iqr),
       TRUE ~ y)) %>%
     dplyr::select(lower, upper, replacement) %>%
     tibble::as_tibble()
diff --git a/man/detect_outlr.Rd b/man/detect_outlr.Rd
index 4a3c24c0..d832cd6d 100644
--- a/man/detect_outlr.Rd
+++ b/man/detect_outlr.Rd
@@ -7,7 +7,7 @@
 detect_outlr(
   x = seq_along(y),
   y,
-  methods = tibble(method = "rm", args = list(list()), abbr = "rm"),
+  methods = tibble::tibble(method = "rm", args = list(list()), abbr = "rm"),
   combiner = c("median", "mean", "none")
 )
 }
diff --git a/tests/testthat.R b/tests/testthat.R
new file mode 100644
index 00000000..b26d274b
--- /dev/null
+++ b/tests/testthat.R
@@ -0,0 +1,4 @@
+library(testthat)
+library(epiprocess)
+
+test_check("epiprocess")
diff --git a/tests/testthat/test-outliers.R b/tests/testthat/test-outliers.R
new file mode 100644
index 00000000..acdc2a35
--- /dev/null
+++ b/tests/testthat/test-outliers.R
@@ -0,0 +1,7 @@
+test_that("detect_outlr throws error with duplicate x", {
+  expect_error(detect_outlr(x = c(1, 2, 3, 3, 4), y = 1:5))
+})
+
+test_that("detect_outlr throws error with length(x) != length(y)", {
+  expect_error(detect_outlr(x = 1:3, y = 1:5))
+})
diff --git a/vignettes/outliers.Rmd b/vignettes/outliers.Rmd
index 1f91f17d..22bae575 100644
--- a/vignettes/outliers.Rmd
+++ b/vignettes/outliers.Rmd
@@ -75,7 +75,8 @@ detection_methods = bind_rows(
          abbr = "rm"),
   tibble(method = "stl",
          args = list(list(detect_negatives = TRUE,
-                          detection_multiplier = 2.5)),
+                          detection_multiplier = 2.5,
+                          seasonal_period = 7)),
          abbr = "stl_seasonal"),
   tibble(method = "stl",
          args = list(list(detect_negatives = TRUE,