Spoiler effect

The spoiler effect is a term to describe the effect a candidate can have on a close election, in which their candidacy results in the election being won by a candidate dissimilar to them, rather than a candidate similar to them.

Mathematical definitions

Possible mathematical definitions for the spoiler effect include failure of independence of irrelevant alternatives (IIA) and vote-splitting.

Related Topics:
Independence of irrelevant alternatives - Vote-splitting

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Independence of irrelevant alternatives

Arrow's impossibility theorem shows that rank-voting systems are unable to satisfy the independence of irrelevant alternatives criterion without exhibiting other undesirable properties as a consequence. However, different voting systems are affected to a greater or lesser extent by IIA failure. For example, instant runoff voting is considered to have less frequent IIA failure than First Past the Post, and Condorcet methods are considered to have less frequent IIA failure than instant runoff voting. The local independence of irrelevant alternatives criterion is a weakened version of IIA that some ranked ballot methods pass.

Related Topics:
Arrow's impossibility theorem - Independence of irrelevant alternatives - Voting system - Instant runoff voting - First Past the Post - Condorcet methods - Local independence of irrelevant alternatives

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Vote-splitting

Voting methods that fail independence of clones may suffer from vote-splitting, teaming, or crowding. Vote-splitting happens when adding similar or clone candidates decreases the chance of any of them winning. Methods that suffer from vote-splitting include First Past the Post and two-round runoff.

Related Topics:
Independence of clones - First Past the Post - Two-round runoff

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