<p>We study the problem of aggregating individual preferences into a collective ranking without ties (a linear order) over a finite set of alternatives. We introduce a framework that constructs such rankings from supermajority rules, defined by a threshold parameter <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\alpha \in \left[ 1/2,1\right) \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>α</mi> <mo>∈</mo> <mfenced close=")" open="["> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> <mo>,</mo> <mn>1</mn> </mfenced> </mrow> </math></EquationSource> </InlineEquation>. For each <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\alpha \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>α</mi> </math></EquationSource> </InlineEquation>, we consider the binary relation of defeats from pairwise comparisons and apply two closure operations—the transitive closure and the Suzumura-consistent closure—to eliminate cycles. We then study the sets of linear orders that respect these relations; that is, linear orders that preserve all strict defeats while extending them to a complete ranking. To avoid the arbitrariness of fixing a single <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\alpha \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>α</mi> </math></EquationSource> </InlineEquation>, we take the intersection across all <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\alpha \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>α</mi> </math></EquationSource> </InlineEquation>. This yields two sets of linear orders: the <i>T</i>-order set (from the transitive closure) and the <i>S</i>-order set (from the Suzumura-consistent closure), both always nonempty, the former is a subset of the latter, and jointly capturing the full spectrum of supermajority rules. We show that this perspective unifies prominent methods: the Schulze method coincides with the <i>T</i>-order set, the Split Cycle method with the <i>S</i>-order set, and every outcome of the Ranked Pairs method lies within the <i>S</i>-order set. Moreover, every linear order in the <i>S</i>-order set satisfies the extended Condorcet criterion and the strong Pareto principle. Our results thus place these methods within a common supermajority-based framework and clarify their relationships.</p>

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Respecting linear orders for supermajority rules

  • Yasunori Okumura

摘要

We study the problem of aggregating individual preferences into a collective ranking without ties (a linear order) over a finite set of alternatives. We introduce a framework that constructs such rankings from supermajority rules, defined by a threshold parameter \(\alpha \in \left[ 1/2,1\right) \) α 1 / 2 , 1 . For each \(\alpha \) α , we consider the binary relation of defeats from pairwise comparisons and apply two closure operations—the transitive closure and the Suzumura-consistent closure—to eliminate cycles. We then study the sets of linear orders that respect these relations; that is, linear orders that preserve all strict defeats while extending them to a complete ranking. To avoid the arbitrariness of fixing a single \(\alpha \) α , we take the intersection across all \(\alpha \) α . This yields two sets of linear orders: the T-order set (from the transitive closure) and the S-order set (from the Suzumura-consistent closure), both always nonempty, the former is a subset of the latter, and jointly capturing the full spectrum of supermajority rules. We show that this perspective unifies prominent methods: the Schulze method coincides with the T-order set, the Split Cycle method with the S-order set, and every outcome of the Ranked Pairs method lies within the S-order set. Moreover, every linear order in the S-order set satisfies the extended Condorcet criterion and the strong Pareto principle. Our results thus place these methods within a common supermajority-based framework and clarify their relationships.