[EM] Margins vs. Winning Votes

[Juho Laatu]

Hello Dave,

On Jul 22, 2005, at 17:25, Dave Ketchum wrote:

> On Wed, 20 Jul 2005 21:36:00 +0300 Juho Laatu wrote:

>> - In raking based real life elections it seems to be quite common 
>> that voters don't give full rankings. This example has only three 
>> candidates and therefore full rankings could be quite common. But the 
>> election could have also considerably more than three candidates, in 
>> which case partial rankings probably would be quite common. It is 
>> probable that ranking candidates of competing party is less common 
>> than ranking candidates of ones own party (just like in this 
>> example).
>
>
> Partial rankings ARE appropriate - you properly do not rank when you 
> do not know or care which of the leftovers are better.

I agree. I just try to study when partial rankings are used. In the 
example candidates of the competing party were unknown to some of the 
voters, which led to leaving them out of the ballots. The example had 
only three candidates (to keep it simple). In real elections number of 
candidates may be also much higher. In this case many voters certainly 
would find ranking all of them (especially the unknown ones) tedious. 
Partial rankings are thus very natural, and leaving unknown and 
uninteresting candidates and unlikely winners unranked are probably 
some of the most typical cases.

>> Now, what if some of  the the 20 C supporters (C>B voters) would note 
>> the weak position of C before the election and decide to vote 
>> strategically C>A>B.
>
>
> The word "strategically" turns me off.  For whatever reason, they are 
> switching between C>B>A (with A's position implied) and C>A>B - 
> between B is preferable to A to A is preferable to B.
>      Not surprising that B no longer wins.
>      Not surprising that C wins in WV - C retains its ranking while B 
> becomes less desirable.
>      Perhaps less surprising that A wins in margins - these voters 
> were SUCCESSFUL in controlling outcome between A and B (though they 
> were unable to cause C to win).

I made the assumption (although I didn't mention this in my mail) that 
the original votes in the example were sincere, so the changed votes 
were not in line with the voters' true preferences. I don't know in 
what sense word "strategically" turns you off, but for me the problem 
is the unwanted property of a voting system where some voters may make 
their favourite win by strategical voting. And what makes the 
"strategic problems" of this example worse is that it seems that as a 
result of the strategy the winner is a candidate that voters didn't 
seem to value very much (different opinions on the strength of C may 
however exist).

Technically the findings are not surprising but at lest for me the fact 
that this insincere voting technique can be applied in some pretty 
normal election situations with reasonable probability of success and 
low risk (and when a "not so good" candidate wins as a result) sounds 
like an unwanted feature that should be studied. Maybe the existence of 
this problem is not a big surprise since it is known that Condorcet 
methods are not strategy free but there is maybe some element of 
surprise in that it is the winning votes that seem to suffer from this 
problem. It seems to be a general belief that winning votes are less 
vulnerable to strategies than margins. I presented this example to shed 
light to the winning votes vs. margins discussion also from the 
opposite angle. I'm eager to hear if the experts can dig out some 
conclusions out of this example or estimate its importance.

Yours,
Juho