Tactical voting: Difference between revisions
Add information about Myerson-Weber strategy from Wikipedia.
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== Rational voter model ==
Academic analysis of tactical voting is based on the rational voter model, derived from [[w:rational choice theory]]. In this model, voters are ''short-term instrumentally rational''. That is, voters are only voting in order to make an impact on one election at a time (not, say, to build the political party for next election); voters have a set of sincere preferences, or utility rankings, by which to rate candidates; voters have some knowledge of each other's preferences; and voters understand how best to use tactical voting to their advantage. The extent to which this model resembles real-life elections is the subject of considerable academic debate.
=== Predisposition to sincerity ===
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Blais and Nadeau use a two-step analysis procedure to argue that 30% of the voters who would have benefited from strategic voting in the 1988 Canadian election actually did vote strategically.<ref name="Blais Nadeau 1996 pp. 39–52">{{cite journal | last=Blais | first=André | last2=Nadeau | first2=Richard | title=Measuring strategic voting: A two-step procedure | journal=Electoral Studies | publisher=Elsevier BV | volume=15 | issue=1 | year=1996 | issn=0261-3794 | doi=10.1016/0261-3794(94)00014-x | pages=39–52}}</ref> They furthermore reason that tactical voting is more prevalent if the voters have only a weak intensity of preference for their first choice over their second, or if the election is a close race between their second and third choice.
=== Myerson-Weber strategy ===
One type of general rational voter strategy is given by Myerson and Weber.<ref>{{cite journal |jstor = 2938959|title = A Theory of Voting Equilibria|journal = The American Political Science Review|volume = 87|issue = 1|pages = 102–114|last1 = Myerson|first1 = Roger B.|last2 = Weber|first2 = Robert J.|year = 1993|doi = 10.2307/2938959|url = http://www.kellogg.northwestern.edu/research/math/papers/782.pdf|hdl = 10419/221141|hdl-access = free}}</ref> It consists of each voter estimating how likely it is that pairs of candidates are going to be tied, and then voting to optimally break the most likely ties.
The model assumes that the voter's utility depends only on who wins, not (for instance) whether a losing candidate the voter supports is seen to have put up a good fight.
For a [[weighted positional system]], the strategy can be formally described as follows. Let there be ''k'' candidates and define
: ''v''<sub>''i''</sub> = the number of points to be voted for candidate ''i''
: ''u''<sub>''i''</sub> = the voter's gain in utility if candidate ''i'' wins the election
: ''p''<sub>''ij''</sub> = the (voter's perceived) '''pivot probability''' that candidates ''i'' and ''j'' will be tied for the most total points to win the election.
Then the voter's '''prospective rating''' for a candidate ''i'' is defined as:
: <math>R_i = \sum_{j \neq i} \; p_{ij} \cdot (u_i - u_j)\,</math>
The gain in expected utility for a given vote is given by:
: <math>G(p,v,u) = \sum_{i=1}^k \; v_i \cdot R_i\,</math>
Formal strategies like the Myerson-Weber can be incorporated into voting methods to produce a [[declared strategy voting]] method. For instance, [[Range voting]] can be thus augmented to produce [[Strategy Advisor based on Randomized Voter Order|SARVO-Range]].
== Pre-election influence ==
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