FA-6295 / Statistics / Member archive

Jaccard set similarity · case 05

Adding input lengths double-counts the intersection and duplicates.

Member previewVariant 5 · 3 implementations · 4 checks per implementation

Case contract

Integer finite observations and equal lengths for paired samples. Counts and weights are nonnegative. Rational results use reduced Fraction strings. Empty or undefined statistics return None where shown. Duplicates do not affect either set. Two empty sets have similarity one; exactly one empty set has zero. Exact operational definition: str(Fraction(len(set(a)&set(b)),len(set(a)|set(b)))) if a or b else "1"

Why this case matters

Small exact fixtures expose this error without platform timing, external services, or probabilistic observations. Statistical estimators results depend on the stated convention.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
fixture 1: ([1, 2], [2, 3])	`"1/4"`	`"1/3"`	Failed

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