Equal Vote Criterion

The Equal Vote Criterion or Equality Criterion is a voting method criterion which requires that a voting method ensure that every voter may cast a vote which is as powerful as a vote cast by any other voter. Voting methods which pass the Equal Vote Criterion do not exhibit vote-splitting or the "Spoiler Effect," ensuring that every vote can cast an equally weighted vote.

In general cardinal voting methods pass the Equal Vote Criterion, including STAR voting, Approval voting, and Score voting. Many Condorcet methods also pass the criterion including most Condorcet methods which can be calculated only with the pairwise counting matrix, as well as most Condorcet-cardinal hybrids.

Some voting methods go further and actually guarantee an Equal Vote, assuming that the ballot is not left blank. STAR Voting, which is binary in the final round, guarantees that every vote cast is equally weighted, regardless of the initial scores given. Approval voting also guarantees an Equal Vote. If Score voting ballots are normalized to ensure that a minimum and maximum score is always given then Score voting guarantees an Equal Vote.

Choose-One Plurality Voting (First Past the Post) and Instant Runoff Voting (often referred to as Ranked Choice Voting) do not satisfy the Equal Vote Criterion.

Any voting method will satisfy the Equal Vote Criterion in elections with two candidates only.

Legal Application
The Equal Vote Criterion is directly related to the concept of an Equally Weighted Vote and the concept of One Person, One Vote. In 1964, Wesberry v. Sanders, The U.S. Supreme Court declared that equality of voting - one person, one vote - means that "the weight and worth of the citizens' votes as nearly as is practicable must be the same."

The 1964, Wesberry v. Sanders case cited above addressed Gerrymandering. In the case of district lines it's impossible to ensure that elections will not favor one faction or the other over time as populations grow and change, but it is "practicable" to prevent and mitigate this phenomena. However in the cases of both vote-splitting and the Electoral Collage achieving a perfectly Equally Weighted Vote is fully possible.

Equal Vote Criterion Definition
Any voting method or election which passes the Test of Balance passes the Equal Vote Criterion and can be said to guarantee an Equally Weighted Vote. The test of balance is defined as the following "Any way I vote, you should be able to vote in an equal and opposite fashion. Our votes should be able to cancel each other’s out." In other words, if an election was tied and one person cast a vote, there must always be a way to cast an opposite vote which would bring the election back to tied.

BTernaryTau has developed two possible formalizations of this criterion, the cancellation criterion and the opposite cancellation criterion.