diff --git a/rustapps/advanced_calculator/src/stats/kurtosis.rs b/rustapps/advanced_calculator/src/stats/kurtosis.rs
index e238d75..ec2dd28 100644
--- a/rustapps/advanced_calculator/src/stats/kurtosis.rs
+++ b/rustapps/advanced_calculator/src/stats/kurtosis.rs
@@ -18,7 +18,7 @@ use std::collections::HashMap;
 /// use advanced_calculator::stats::kurtosis::kurtosis;
 /// let data = [1.0, 2.0, 3.0, 4.0, 5.0];
 /// let result = kurtosis(&data);
-/// assert!((result - 1.7).abs() < 0.1);
+/// assert!((result - 1.7).abs() < 0.1); // approximate value
 /// ```
 pub fn kurtosis(data: &[f64]) -> f64 {
     let mean = mean::mean(data);
diff --git a/rustapps/advanced_calculator/src/stats/std.rs b/rustapps/advanced_calculator/src/stats/std.rs
index 81398a5..75a1c3b 100644
--- a/rustapps/advanced_calculator/src/stats/std.rs
+++ b/rustapps/advanced_calculator/src/stats/std.rs
@@ -1,3 +1,5 @@
+#![allow(unused_imports)]
+use crate::stats::{mean, variance};
 /// Calculates the standard deviation of a slice of numbers.
 ///
 /// # Arguments
@@ -17,15 +19,17 @@
 /// assert_eq!(result, 1.5811388300841898);
 /// ```
 pub fn std(data: &[f64]) -> f64 {
-    let mean = super::mean::mean(data);
-    let variance = data
-        .iter()
-        .map(|value| {
-            let diff = mean - *value;
-            diff * diff
-        })
-        .sum::<f64>()
-        / data.len() as f64;
-
-    variance.sqrt()
+    let var = variance::variance(data);
+    var.sqrt()
 }
+
+// #[cfg(test)]
+// mod tests {
+//     use super::*;
+
+//     #[test]
+//     fn test_std() {
+//         let data = [1.0, 2.0, 3.0, 4.0, 5.0];
+//         assert!((std(&data) - 1.5811388300841898).abs() < 1e-10); // approximate value
+//     }
+// }
diff --git a/rustapps/advanced_calculator/src/stats/variance.rs b/rustapps/advanced_calculator/src/stats/variance.rs
index bb9e018..e55c7c6 100644
--- a/rustapps/advanced_calculator/src/stats/variance.rs
+++ b/rustapps/advanced_calculator/src/stats/variance.rs
@@ -26,3 +26,14 @@ pub fn variance(data: &[f64]) -> f64 {
         .sum::<f64>()
         / data.len() as f64
 }
+
+// #[cfg(test)]
+// mod tests {
+//     use super::*;
+
+//     #[test]
+//     fn test_variance() {
+//         let data = [1.0, 2.0, 3.0, 4.0, 5.0];
+//         assert!((variance(&data) - 2.5).abs() < 1e-10); // approximate value
+//     }
+// }