Tideman alternative method

Tideman's Alternative Method, also called Alternative Smith or Alternative Schwartz, is an electoral system developed by Nicolaus Tideman which selects a single winner using votes that express preferences.

This method is Smith-efficient, making it a kind of Condorcet method. It uses instant-runoff voting for cycle resolution.

Procedure


The Alternative Smith procedure is as follows:
 * 1) Eliminate all candidates outside the Smith set.
 * 2) If there is more than one candidate remaining, eliminate the last-place candidate as in IRV.
 * 3) Repeat the procedure until there is only one candidate left.

Strategy-resistance
Alternative Smith strongly resists both strategic nomination and strategic voting by political parties or coalitions (although like every system, it can still be manipulated in some situations). The Smith and runoff components of Smith-runoff cover up each other's weaknesses:


 * 1) Smith-efficient methods are difficult for any coalition to manipulate, because no majority-strength coalition will have an incentive to remove a Condorcet winner: if most voters prefer A to B,  A can already defeat B.
 * 2) * This reasoning does not apply to situations with a Condorcet cycle, however.
 * 3) * While Condorcet cycles are rare in practice with honest voters, burial (ranking a strong rival last, below weak opponents) can often create a false cycle.
 * 4) Instant runoff voting is resistant to burial, because it is only based on each voter's top preference in any given round. This means that burial strategies effective against the Smith-elimination step are not effective against the instant runoff step.
 * 5) * On the other hand, instant-runoff voting is highly vulnerable to a lesser evil (decapitation) strategy: defeating a greater evil requires voters to rank a strong candidate first, rather than express their sincere beliefs.
 * 6) * However, if such a candidate exists (with majority support), they will usually be a Condorcet winner, and elected in the first round.

The combination of these two methods creates a highly-resistant system.

Spoiler effects
Alternative Smith fails independence of irrelevant alternatives, meaning it can sometimes be affected by spoiler candidates. However, the method adheres to a weaker property that eliminates most spoilers, sometimes called independence of Smith-dominated alternatives (ISDA). This method states that if one candidate (X) wins an election, and a new alternative (Y) is added, X will still win the election as long as Y is not in the highest-ranked cycle.

Comparison table
The following table compares Alternative Smith with other single-winner election methods: