[EM] Why I Prefer IRV to Condorcet

[Chris Benham]

Greg,
I generally liked your essay. I rate IRV as the best of the single-winner methods that
meet Later-no-Harm, and a good method (and a vast improvement on FPP).

But I think you made a couple of technical errors.

"However, because bullet voting can help and never backfire against one's top choice under
Condorcet, expect every campaign with a shot at winning to encourage its supporters to 
bullet vote. "

Bullet voting can "backfire against one's top choice under Condorcet" because Condorcet
methods, unlike IRV, fail Later-no-Help. 

http://groups.yahoo.com/group/election-methods-list/files/wood1996.pdf

In this 1996 Douglas Woodall paper, see "Election 6" and the accompanying discussion on
page 5/6 of the pdf (labelled on the paper as "Page 13").

Quoting again from your paper:
"As mentioned, every voting system is theoretically vulnerable to strategic manipulation, and IRV 
is no exception. However, under IRV, there is no strategy that can increase the likelihood of 
electing one's first choice beyond the opportunity offered by honest rankings. While there are 
strategies for increasing the chances of less preferred candidates under IRV, like push-over, 
they are counter-intuitive."

The Push-over strategy is certainly not limited to improving the chance of electing a "lower 
[than first] choice". Say sincere is:

49: A 
27: B>A
24: C>B

B is the IRV winner, but if  4-21 (inclusive) of the A voters change to C or C>? then the winner
changes to A.

But as you say the strategy isn't "intuitive" , and backfires if too many of the A supporters try it.
Some IRV opponents claim to like Top-Two Runoff, but that is more vulnerable to Push-over 
than IRV (because the strategists can support their sincere favourite in the second round).

The quite intuitive strategy that IRV is vulnerable to is Compromise, like any other method that
meets Majority. But voters' incentive to compromise (vote one's front-runner lesser-evil in first
place to reduce the chance of front-runner greater-evil winning) is generally vastly vastly less
than it is under FPP.

(There are methods that meet both Majority and Favourite Betrayal, and in them compromisers
can harmlessly vote their sincere favourites in equal-first place.)

But some Condorcet advocates are galled  by the Compromise incentive that can exist where
there is a sincere CW who is not also a sincere Mutual Dominant Third winner.

49: A>B
02: B>A
22: B
27: C>B

On these votes B is the CW, but IRV elects A.  If the C>B voters change to B then B will be 
the voted majority favourite, so of course IRV like Condorcet methods and FPP will elect B.

Chris Benham

 
Greg wrote (Wed.Nov.19, 2008):
I have written up my reasons for preferring IRV over Condorcet methods
in an essay, the current draft of which is available here:
  http://www.gregdennis.com/voting/irv_vs_condorcet.html

I welcome any comments you have.

Thanks,
Greg



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