FA-329 / Floating-point arithmetic / Member archive

Subtracting two large moments destroys a small variance · case 04

A nonconstant sample reports zero or an inaccurate variance after its mean is shifted far from zero.

Member previewVariant 4 · 3 implementations · 7 checks per implementation

Case contract

Return statistics.pvariance for a nonempty sequence of finite floats; return None for empty input. This is population variance, so the denominator is n rather than n-1.

Why this case matters

Variance should not change when all observations receive the same offset. Large baselines with small spreads are a direct regression check for cancellation-prone moment formulas.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
unit spread on large baseline	`0.0`	`0.6666666666666666`	Failed

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This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

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