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Gibbard's 1978 theorem states that a nondeterministic voting method is only strategyproof if it's a mixture of unilateral and duple rules. For instance, the rule that flips a coin and chooses a random dictator if the coin lands on heads, or chooses the pairwise winner between two random candidates if the coin lands on tails, is strategyproof.
Gibbard's proof of the theorem is more general and covers processes of collective decision that may not be ordinal, such as cardinal voting. [note 1] Gibbard's 1978 theorem and Hylland's theorem are even more general and extend these results to non-deterministic processes, where the outcome may depend partly on chance; the Duggan–Schwartz ...
Strategic or tactical voting is voting in consideration of possible ballots cast by other voters in order to maximize one's satisfaction with the election's results. [1] Gibbard's theorem shows that no voting system has a single "always-best" strategy, i.e. one that always maximizes a voter's satisfaction with the result, regardless of other ...
Gibbard's theorem shows that any strategyproof game form (i.e. one with a dominant strategy) with more than two outcomes is dictatorial. The Gibbard–Satterthwaite theorem is a special case showing that no deterministic voting system can be fully invulnerable to strategic voting in all circumstances, regardless of how others vote.
On a rated ballot, the voter may rate each choice independently. An approval voting ballot does not require ranking or exclusivity. Rated, evaluative, [1] [2] graded, [1] or cardinal voting rules are a class of voting methods that allow voters to state how strongly they support a candidate, [3] by giving each one a grade on a separate scale.
Black proved that by replacing unrestricted domain with single-peaked preferences in Arrow's theorem removes the impossibility: there are Pareto-efficient non-dictatorships that satisfy the "independence of irrelevant alternatives" criterion. However, Black's 1948 proof was published before Arrow's impossibility theorem was published in 1950 ...
The median voter theorem relates to ranked voting mechanisms, in which each agent reports his/her full ranking over alternatives. The theorem says that, if the agents' preferences are single-peaked , then every Condorcet method always selects the candidate preferred by the median voter (the candidate closest to the voter whose peak is the ...
In social choice theory, a dictatorship mechanism is a degenerate voting rule or mechanism where the result depends on only one person's preferences, without considering any other voters. A serial dictatorship is similar, but also designates a series of "backup dictators", who break ties in the original dictator's choices when the dictator is ...