From 285147d52b9fab888bf8f0c09d7cbee40f3e8e27 Mon Sep 17 00:00:00 2001
From: Palash Tyagi <23239946+Magnus167@users.noreply.github.com>
Date: Tue, 15 Jul 2025 01:00:03 +0100
Subject: [PATCH] Refactor variance functions to distinguish between population
 and sample variance

---
 src/compute/stats/descriptive.rs | 53 ++++++++++++++++++++++----------
 1 file changed, 37 insertions(+), 16 deletions(-)

diff --git a/src/compute/stats/descriptive.rs b/src/compute/stats/descriptive.rs
index 7dedee8..d52ab62 100644
--- a/src/compute/stats/descriptive.rs
+++ b/src/compute/stats/descriptive.rs
@@ -14,17 +14,29 @@ pub fn mean_horizontal(x: &Matrix<f64>) -> Matrix<f64> {
     Matrix::from_vec(x.sum_horizontal(), x.rows(), 1) / n
 }
 
-pub fn variance(x: &Matrix<f64>) -> f64 {
+fn population_or_sample_variance(x: &Matrix<f64>, population: bool) -> f64 {
     let m = (x.rows() * x.cols()) as f64;
     let mean_val = mean(x);
     x.data()
         .iter()
         .map(|&v| (v - mean_val).powi(2))
         .sum::<f64>()
-        / m
+        / if population { m } else { m - 1.0 }
 }
 
-fn _variance_axis(x: &Matrix<f64>, axis: Axis) -> Matrix<f64> {
+pub fn population_variance(x: &Matrix<f64>) -> f64 {
+    population_or_sample_variance(x, true)
+}
+
+pub fn sample_variance(x: &Matrix<f64>) -> f64 {
+    population_or_sample_variance(x, false)
+}
+
+fn _population_or_sample_variance_axis(
+    x: &Matrix<f64>,
+    axis: Axis,
+    population: bool,
+) -> Matrix<f64> {
     match axis {
         Axis::Row => {
             // Calculate variance for each column (vertical variance)
@@ -39,7 +51,7 @@ fn _variance_axis(x: &Matrix<f64>, axis: Axis) -> Matrix<f64> {
                     let diff = x.get(r, c) - mean_val;
                     sum_sq_diff += diff * diff;
                 }
-                result_data[c] = sum_sq_diff / num_rows;
+                result_data[c] = sum_sq_diff / (if population { num_rows } else { num_rows - 1.0 });
             }
             Matrix::from_vec(result_data, 1, x.cols())
         }
@@ -56,30 +68,39 @@ fn _variance_axis(x: &Matrix<f64>, axis: Axis) -> Matrix<f64> {
                     let diff = x.get(r, c) - mean_val;
                     sum_sq_diff += diff * diff;
                 }
-                result_data[r] = sum_sq_diff / num_cols;
+                result_data[r] = sum_sq_diff / (if population { num_cols } else { num_cols - 1.0 });
             }
             Matrix::from_vec(result_data, x.rows(), 1)
         }
     }
 }
 
-pub fn variance_vertical(x: &Matrix<f64>) -> Matrix<f64> {
-    _variance_axis(x, Axis::Row)
+pub fn population_variance_vertical(x: &Matrix<f64>) -> Matrix<f64> {
+    _population_or_sample_variance_axis(x, Axis::Row, true)
 }
-pub fn variance_horizontal(x: &Matrix<f64>) -> Matrix<f64> {
-    _variance_axis(x, Axis::Col)
+
+pub fn population_variance_horizontal(x: &Matrix<f64>) -> Matrix<f64> {
+    _population_or_sample_variance_axis(x, Axis::Col, true)
+}
+
+pub fn sample_variance_vertical(x: &Matrix<f64>) -> Matrix<f64> {
+    _population_or_sample_variance_axis(x, Axis::Row, false)
+}
+
+pub fn sample_variance_horizontal(x: &Matrix<f64>) -> Matrix<f64> {
+    _population_or_sample_variance_axis(x, Axis::Col, false)
 }
 
 pub fn stddev(x: &Matrix<f64>) -> f64 {
-    variance(x).sqrt()
+    population_variance(x).sqrt()
 }
 
 pub fn stddev_vertical(x: &Matrix<f64>) -> Matrix<f64> {
-    variance_vertical(x).map(|v| v.sqrt())
+    population_variance_vertical(x).map(|v| v.sqrt())
 }
 
 pub fn stddev_horizontal(x: &Matrix<f64>) -> Matrix<f64> {
-    variance_horizontal(x).map(|v| v.sqrt())
+    population_variance_horizontal(x).map(|v| v.sqrt())
 }
 
 pub fn median(x: &Matrix<f64>) -> f64 {
@@ -180,7 +201,7 @@ mod tests {
         assert!((mean(&x) - 3.0).abs() < EPSILON);
 
         // Variance
-        assert!((variance(&x) - 2.0).abs() < EPSILON);
+        assert!((population_variance(&x) - 2.0).abs() < EPSILON);
 
         // Standard Deviation
         assert!((stddev(&x) - 1.4142135623730951).abs() < EPSILON);
@@ -209,7 +230,7 @@ mod tests {
         assert!((mean(&x) - 22.0).abs() < EPSILON);
 
         // Variance should be heavily affected by outlier
-        assert!((variance(&x) - 1522.0).abs() < EPSILON);
+        assert!((population_variance(&x) - 1522.0).abs() < EPSILON);
 
         // Standard Deviation should be heavily affected by outlier
         assert!((stddev(&x) - 39.0128183970461).abs() < EPSILON);
@@ -258,12 +279,12 @@ mod tests {
         let x = Matrix::from_vec(data, 2, 3);
 
         // cols: {1,4}, {2,5}, {3,6} all give 2.25
-        let vv = variance_vertical(&x);
+        let vv = population_variance_vertical(&x);
         for c in 0..3 {
             assert!((vv.get(0, c) - 2.25).abs() < EPSILON);
         }
 
-        let vh = variance_horizontal(&x);
+        let vh = population_variance_horizontal(&x);
         assert!((vh.get(0, 0) - (2.0 / 3.0)).abs() < EPSILON);
         assert!((vh.get(1, 0) - (2.0 / 3.0)).abs() < EPSILON);
     }