[EM] serious strategy problem in Condorcet, but not in IRV?

[Markus Schulze]

Dear James Green-Armytage,

I wrote (19 Aug 2003):
> I would point to the fact that IRV is vulnerable to "pushing over"
> (i.e. ranking a weak candidate higher than one's preferred candidate)
> while those methods that meet monotonicity aren't.

You wrote (19 Aug 2003):
> Do you mean something like this: (IRV is the method being used)
>
> Sincere preferences
> 47: Bush
> 6: Gore > Bush
> 21: Gore > Nader
> 26: Nader > Gore
>
> Gore wins IRV.
>
> Strategically altered preferences (2 Bush voters switch to Nader > Bush)
>
> 45: Bush
> 6: Gore > Bush
> 21: Gore > Nader
> 26: Nader > Gore
> 2: Nader > Bush
>
> Gore is eliminated first, and Bush wins. Is this a good example of
> pushing over?

Yes! But it would have been better if you had used symbols instead
of names. Now, many readers will reply that your example is quite
unrealistic since it is quite improbable that Nader gets 26% of the
first preferences.

******

You wrote (19 Aug 2003):
> Could it be possible to design a version of Condorcet that is more
> strategy-proof than beatpath or ranked pairs?

That depends on what you mean with "strategy-proofness". For example:
Mike Ossipoff and Russ Paielli consider invulnerability to "compromising"
(i.e. ranking a candidate insincerely higher to make him win) to be not
a strategical problem. Therefore, they consider FPP to be strategy-proof.
The only reason why they reject FPP is that FPP can find an "obviously
wrong winner".

Russ Paielli wrote (6 Aug 2003):
> Plurality is also "extremely difficult to manipulate" in this way,
> isn't it? What kind of "offensive strategy" could you possibly use 
> in plurality? None that I can think of.
>
> (...)
>
> The one and only possible way of helping the Libertarian (or any other
> candidate) win is to vote for him. But then you are voting sincerely,
> hence you are not "manipulating" anything. Plurality is therefore
> beyond manipulation. 

What I wanted to say is: From a given point of view, it is rather
subjective which strategical problem (e.g. truncating, burying,
compromising, pushing over) is more offensive, more undesirable,
more drastic, more implausible, more counter-intuitive, etc..

Markus Schulze