Wikipedia: History of Runoff voting

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Monotonicity

If alternative X wins, and the only changes to the ballots are changes that increase the ranking of X, then X should still win. That is the monotonicity crierion.

All runoff voting methods violate the monotonicity criterion, including this one. For example, let's assume the voters' sincere preferences remain static between rounds as follows:

8 A>B>C
9 B>C>A
11 C>A>B
5 A>C>B

A B C
1st Round: 13 9 11
2nd Round: 13 - 20
Outcome: C>A>B

Then say the A>C>B voters change to C>A>B in order to feel like winners, which means that the only change in the profile is that C, the original winner, gets even more support than she had before. C no longer wins first place.

8 A>B>C
9 B>C>A
16 C>A>B

A B C
1st Round: 8 9 16
2nd Round: - 17 16
Outcome: B>C>A

See also: Instant runoff voting

	Revision 2 . . November 1, 2001 1:56 am by (logged).233.140.xxx
	Revision 1 . . September 17, 2001 1:39 am by DanKeshet [initial revision]