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Return None from Empirical::variance for a single sample #468

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Merged
YeungOnion merged 1 commit into from
Sep 7, 2026
Merged

Return None from Empirical::variance for a single sample #468

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9 changes: 8 additions & 1 deletion src/distribution/empirical.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -243,7 +243,8 @@ impl Distribution<f64> for Empirical {
}

fn variance(&self) -> Option<f64> {
if self.data.is_empty() {
// The `n - 1` denominator is zero for a single sample, so the sample variance is undefined.
if self.sum < 2 {
None
} else {
Some(self.var / (self.sum as f64 - 1.))
Expand Down Expand Up @@ -345,6 +346,12 @@ mod tests {
let dist = Empirical::from_iter(vec![]);
assert!(dist.variance().is_none());

// A single sample has no `n - 1` denominator; the sample variance is undefined.
let one = Empirical::from_iter(vec![4.0]);
assert!(one.variance().is_none());
assert!(one.std_dev().is_none());
test_var_for_samples(2.0, vec![1.0, 3.0]);

test_var_for_samples(0.0, vec![4.0; 100]);
test_var_for_samples(0.0, vec![-0.2; 100]);
test_var_for_samples(190.36666666666667, vec![21.3, 38.4, 12.7, 41.6]);
Expand Down