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Adjusted partition pair agreement: The maximum marginal pair count replaces their average normalization ceiling. · case 03

The reduction disagrees with its explicit aggregation oracle.

Member previewVariant 3 · 3 implementations · 9 checks per implementation

Case contract

Two equal-length integer cluster label arrays describe the same observations. Return exact adjusted Rand index from pair-count contingency reduction. Label names have no numerical meaning. Matching degenerate partitions with zero normalization return "1"; length mismatch returns None.

Why this case matters

Exact bounded examples isolate a reduction defect without floating-point or external-service effects.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression 5	`"-1/14"`	`"-2/23"`	Failed

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