diff --git a/R/central-tendency.R b/R/central-tendency.R
index ba4752f..4da09d3 100644
--- a/R/central-tendency.R
+++ b/R/central-tendency.R
@@ -1,5 +1,4 @@
 # TODO:
-# - Add tests
 # - Add description
 
 #' Geometric Mean (GM)
diff --git a/R/variability.R b/R/variability.R
index e1f6608..0bd8e22 100644
--- a/R/variability.R
+++ b/R/variability.R
@@ -1,5 +1,4 @@
 # TODO:
-# - Add tests
 # - Add description
 
 #' Coefficient of Variation (Cv)
@@ -52,6 +51,9 @@ cv.data.frame <- function(
   )
 }
 
+# TODO:
+# Rewrite in C++
+
 #' @rdname cv
 #' @export
 cv_vec <- function(
@@ -59,6 +61,7 @@ cv_vec <- function(
   na_rm = TRUE,
   ...
 ) {
+  # ugly, might not need
   yardstick::check_numeric_metric(truth, truth, case_weights = NULL)
 
   if (na_rm) {
diff --git a/tests/testthat.R b/tests/testthat.R
index 99bfdc6..883f850 100644
--- a/tests/testthat.R
+++ b/tests/testthat.R
@@ -7,6 +7,7 @@
 # * https://testthat.r-lib.org/articles/special-files.html
 
 library(testthat)
+library(checkmate)
 library(tidyhydro)
 
 test_check("tidyhydro")
diff --git a/tests/testthat/test-kge.R b/tests/testthat/test-kge.R
index 43dd9e7..411d1bc 100644
--- a/tests/testthat/test-kge.R
+++ b/tests/testthat/test-kge.R
@@ -81,3 +81,12 @@ test_that("Internal function works as expected", {
       kge_cpp(ex_dat$obs, ex_dat$pred, na_rm = TRUE, version = "2012")
   )
 })
+
+test_that("Negative values are not allowed in kgelog", {
+  x <- rnorm(100)
+  y <- rnorm(100)
+
+  expect_error(kgelog_vec(x, y))
+  expect_error(kgelog_hi_vec(x, y))
+  expect_error(kgelog_low_vec(x, y))
+})
diff --git a/tests/testthat/test-measures.R b/tests/testthat/test-measures.R
new file mode 100644
index 0000000..54ef69a
--- /dev/null
+++ b/tests/testthat/test-measures.R
@@ -0,0 +1,5 @@
+test_that("Negative values are not allowed in GM", {
+  x <- rnorm(100)
+
+  expect_error(gm_vec(x))
+})
diff --git a/tests/testthat/test-outputs.R b/tests/testthat/test-outputs.R
new file mode 100644
index 0000000..4ce0fbe
--- /dev/null
+++ b/tests/testthat/test-outputs.R
@@ -0,0 +1,424 @@
+# Property-based testing
+# https://www.etiennebacher.com/posts/2024-10-01-using-property-testing-in-r
+
+# Tests to check that no matter the all sorts of numeric values
+# are allowed in functions.
+
+options(
+  quickcheck.tests = 20L,
+  quickcheck.shrinks = 10L,
+  quickcheck.discards = 10L
+)
+
+test_that("GM", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_left_bounded(
+      left = 1L,
+      len = 100L,
+      any_na = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, na_flag) {
+      x <- gm_vec(truth = obs, na_rm = na_flag)
+      df <- gm(data = data.frame(obs), truth = obs, na_rm = na_flag)
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("CV", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, na_flag) {
+      x <- cv_vec(truth = obs, na_rm = na_flag)
+      df <- cv(
+        data = data.frame(obs),
+        truth = obs,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("NSE", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- nse_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- nse(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("KGE", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- kge_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- kge(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("KGE'", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- kge2012_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- kge2012(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("KGElog", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_left_bounded(
+      left = 1,
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_left_bounded(
+      left = 1,
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- kgelog_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- kgelog(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("RMSE", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- rmse_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- rmse(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("MSE", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- mse_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- mse(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("pBIAS", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- pbias_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- pbias(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("PRESS", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- press_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- press(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
+
+test_that("SFE", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    sim = quickcheck::double_(
+      len = 100L,
+      any_na = TRUE,
+      any_nan = TRUE,
+      big_dbl = TRUE
+    ),
+    na_flag = quickcheck::logical_(len = 1L, any_na = FALSE),
+    property = function(obs, sim, na_flag) {
+      x <- sfe_vec(
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+      df <- sfe(
+        data = data.frame(obs, sim),
+        truth = obs,
+        estimate = sim,
+        na_rm = na_flag
+      )
+
+      checkmate::expect_number(x, na.ok = TRUE)
+      checkmate::expect_data_frame(
+        df,
+        types = c("numeric", "character"),
+        min.rows = 1L
+      )
+    }
+  )
+})
diff --git a/tests/testthat/test-system-crash.R b/tests/testthat/test-system-crash.R
index 536690f..644190e 100644
--- a/tests/testthat/test-system-crash.R
+++ b/tests/testthat/test-system-crash.R
@@ -1,6 +1,8 @@
 # Property-based testing
 # https://www.etiennebacher.com/posts/2024-10-01-using-property-testing-in-r
 
+# Tests to ensure that the C++ code wouldn't crash the system
+
 options(
   quickcheck.tests = 20L,
   quickcheck.shrinks = 10L,
@@ -215,3 +217,39 @@ test_that("press", {
     }
   )
 })
+
+test_that("CV", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::any_atomic(any_na = TRUE),
+    na_flag = quickcheck::logical_(any_na = FALSE),
+    property = function(obs, na_flag) {
+      suppressWarnings(
+        try(
+          cv_vec(truth = obs, na_rm = na_flag),
+          silent = TRUE
+        )
+      )
+      expect_true(TRUE)
+    }
+  )
+})
+
+test_that("GM", {
+  skip_if_not_installed("quickcheck")
+
+  quickcheck::for_all(
+    obs = quickcheck::any_atomic(any_na = TRUE),
+    na_flag = quickcheck::logical_(any_na = FALSE),
+    property = function(obs, na_flag) {
+      suppressWarnings(
+        try(
+          gm_vec(truth = obs, na_rm = na_flag),
+          silent = TRUE
+        )
+      )
+      expect_true(TRUE)
+    }
+  )
+})