[EM] The Green scenario, and IRV in the Green scenario, is a new topic here. Hence these additional comments. Clarification of position and why.

[Peter Zbornik]

Kristoffer,

no the example below applies for my two-round proposal as well, thus
rapidly sinking what I previously proposed :o)
Nice to having had done away with the two-round variant of IRV.
Now I don't have to bother about it any more.

For Condorcet I am not sure.
I guess, there might even be a new criterion invented: multiple-round
strategy-proof , but I don't know of any method satisfying this
criterion.

The two-round method would however be suitable when trying out which
of two methods is the best by letting the winners meet in the second
round (like plurality vs. IRV winner), in order to gather political
support, but that's an other topic.

Best regards
Peter Zborník

2013/2/4 Kristofer Munsterhjelm <km_elmet at lavabit.com>:
> On 02/04/2013 09:31 PM, Peter Zbornik wrote:
>>
>> Hi I am afraid a proportional approach in the first round wouldnt
>> work, it opens up for strategic voting.
>> Say we have an election with A, B, C.
>> 45 A
>> 30 B A
>> 25 C B A
>>
>> The first round in a 2-seat election the quota is 34 votes
>> If we would have a two-round proportional election, then B would win
>> in the second round.
>>
>> So A's voters find this out and decide to change their preferences and
>> 10 of the voters of A vote for C
>> So we have
>>
>> 35 A
>> 30 BA
>> 25 CBA
>> 10 CA
>>
>> C and A meet in the second round, where A wins.
>
>
> A one-on-one runoff (i.e. second round), taken on its own, is
> strategy-proof. However, if we imagine the voters never change their
> opinion, then we could build a ranked election system that works as however
> the first round would in reality, then simulates a runoff between the
> winners. This method would, like any other ranked method, be subject to
> Arrow's theorem and to Gibbard-Satterthwaite.
>
> Thus, the runoff can't, as a whole (both rounds considered) be
> strategy-proof. So there will be some kind of strategy. But does a
> proportional first round make it more vulnerable to strategy than a plain
> first round?
>