[EM] Fwd: Ordering defeats in Minimax

[Juho Laatu]

> On 25 Apr 2017, at 06:36, Andrew Myers <andru at cs.cornell.edu> wrote:

> 1. WV: (W1, L1) > (W2, L2) if W1 > W2 or (W1=W2 and L2 > L1)    [currently implemented]
> 2. Margins: (W1, L1) > (W2, L2) if W1 - L1 > W2 - L2
> 3. LV: (W1, L1) > (W2, L2) if L1 < L2 or (L1 = L2 and W1 > W2)

Those functions make sense to me. I would maybe separate the basic WV, margins and LV definitions from the additional tie breaker definitions ("W1=W2 and L2 > L1" and "L1 = L2 and W1 > W2").


> On 25 Apr 2017, at 11:06, Kristofer Munsterhjelm <km_elmet at t-online.de> wrote:

> I'm usually a wv person, but I think Minmax is more classically
> associated with margins. Or perhaps I think that because Juho is here
> and he prefers margins :-)

The strongest argument in favour of margins must be that it is a relatively natural preference function. WV and LV are discontinuous functions and therefore can not really be called natural. Interest in using them comes mainly form strategic defence reasons, not from studying what would be a natural way of measuring strengths of preference. In addition the results that you get close to those discontinuities may appear strange (one additional vote may change the outcome radically).

Preference function between two candidates (A, B) can be given as a function of x and y, where x is the proportion of votes (0..1) ranking A over B, and y is the proportion of votes (0..1) ranking B over A. The values of the function range from 1 (100% of voters rank A over B) to -1 (100% of voters rank B over A). Value 0 refers to a tie.

Margins preference function is x-y
WV preference function is 0 if x=y, x if x>y, -y if y>x
LV preference function is 0 if x=y, 1-y if x>y, x-1 if y>x

WV and LV are not continuous around the x=y line. It is for example strange that in WV result 51-49 is considered a strong victory to A (0.51), while 50-50 gives 0 and 49-51 gives -0.51.

There are also other possible preference functions. One could measure for example the proportion of A>B preferences against all given preferences (A>B or B>A). This one is close to margins in the sense that it can be seen as one natural preference function. It is also smooth and continuous, except that at point 0-0 it is not even defined. The value of result 1-0 is 1, and the value of result 0-1 is -1, which is a quite radical approach.

Margins and proportions have different philosophy with respect to votes that have not given any preference between A and B (indifferent voters). Margins thinks that indifferent voters gave half a vote to support A>B and half a vote to support B>A. Preferences thinks that indifferent voters want to vote in favour of A vs B in same proportion as those voters did that gave a preference. Both approaches can thus be explained as natural approaches to explaining how the indifferent votes should be read.


> On 04/25/2017 03:34 PM, Kevin Venzke wrote:

> The desire to say that a 35-0 win is stronger than a 51-49 win could
> make sense if a strong win in itself was of some value.

This (comparison of different results) is a good approach to evaluating how the indifferent votes should be handled. I already wrote above on how proportions could be considered bad around the 0-0 result. In margins it is a good question if 2-0 should be considered to have equal preference strength to 51-49.

I'll define some additional preference strength functions (P). I start from the assumption that margins philosophy can be taken as the starting point for results where all voters have expressed preference (= no indifferent votes). As a result preference value for 100-0 is 1 (for all preference functions of this group of P functions), i.e. P(100,0)=1, and P(50,50)=0, and P(75,25)=0.5. The preference function is thus linear at that line (y = 1-x). Preference value should also always be 0 at line x=y. More formally, P(x,x)=0 and P(x,1-x)=2x-1. We should require also symmetry, P(x,y)=-P(y,x).

One family of preference functions that meets these criteria is (x-y)*(x+y)^k. We get margins when k=0. We get proportion when k=-1. The next interesting possibility is to use value k=1. This one is interesting because it has some WV like properties without introducing the weird discontinuities of WV. I'll study this function (k=1) a bit more.

(Note that when k=1 the function becomes quite simple, (x-y)*(x+y)^1 = x^2 - y^2.)

First few words about sincere votes. In margins the preference values of 50-0 and 75-25 are equal. Pm(50,0) = Pm(75,25). If one doesn't think this is ideal, one can adjust this relation by adjusting the value of k. When k=1 (P1) we have P1(75,25) = 0.5 (by definition). P1(50,0) = 0.25, Pm(50,0) = 0.5 and Pp(50,0) = 1 (p = proportion). The point of this paragraph is to demonstrate that if you want to have a P that gives best possible results with sincere votes, one can adjust k to reach best performance (within this family of preference functions). One parameter may be enough to address the most typical concerns (?).

What about defence against strategic voting then. In many large public elections such defences may be unnecessary. But for for people that like the WV properties P1 may be of interest also for strategy defence related reasons. It for example gives P1(10,0) = 0.01 while P1(55,45) = 0.1. I will not go further into details although it could be interesting to study the impact of parameters to strategic defence capabilities. The point here is that there are also continuous "natural" preference functions that can reach some of the targets of discontinuous functions like WV.

If one uses parameterized preference functions like the one described above, one can also adjust the parameters (k) so that the function reaches the required strategy resistance, but at the same time deviates from the preferred sincere preference function only as little as possible.


> On 25 Apr 2017, at 11:06, Kristofer Munsterhjelm <km_elmet at t-online.de> wrote:

> On a side note, Minmax can produce a lot of ties if there are few voters
> involved, so sometimes I prefer to break ties by second strongest defeat
> (and then third strongest, fourth strongest, etc). That isn't
> *classical* Minmax, but it shouldn't break any of Minmax's criteria.

I just note that the parameterized versions may also reduce the probability of ties (e.g. if one uses value k=0.00001 instead of k=0).

Juho