Voting Method Criteria & Analysis

Majority Criterion

Introduction

An election method satisfies the majority criterion (MC) if a candidate favored by more than half of voters is guaranteed to win. (Note: all Condorcet methods satisfy the MC.)

Polling Assumption

Significance

The Spoiler Effect

Introduction

In election parlance, a spoiler is a non-winning candidate whose presence on the ballot affects which candidate wins. In mathematical terms, the spoiler effect is when a voting method exhibits failure of a property known as independence of irrelevant alternatives.

Spoilers are possible in all ordinal (“ranked”) voting methods, but not in Score Voting (aka Range Voting). That includes the simplest form of Score Voting, called Approval Voting.

The "Center Squeeze" Effect

Introduction

The “center squeeze” effect is a common type of electoral scenario in which approval voting behaves better than plurality voting, delayed runoff, or instant runoff voting.

Election Example

Say we have the  following voter preferences:

Monotonicity

Introduction

Monotonicity is an election method criterion that requires the following:
Ranking or rating a candidate higher should never cause that candidate to lose, nor should ranking or rating a candidate lower ever cause that candidate to win, assuming all other candidates remain rated or ranked the same.

In other words, an election method is non-monotonic if either of the following is possible:

Later-No-Harm Criterion

Introduction

A voting method satisfies the later-no-harm criterion if a voter cannot cause a more preferred candidate to lose by giving an additional ranking or positive rating to a less preferred candidate.

Passing Later-No-Harm

Consider the following preferences for a few groups of voters, with candidates labeled X to Z.

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