[EM] SFC definition supports partial preference relations

[Jobst Heitzig]

Dear Mike!

you write:
> As for Approval vs ERIRV(fractional), I don't  have a very strong opinion on 
> that issue, because, as I said, SFC is the criterion that I like the most, 
> and it isn't met by ERIRV.

and your definition of SFC included:
> Because SFC is about the freedom to sincerely vote all of one's
> preferences, its first requirement is that the voting system
> allow a voter to express as many prefernces as he/she wishes.
> It's possible to allow that, for reasonable-size candidate sets,
> with any implementation, but the method's definition should allow
> that right even if a particular implementation limits the length
> of a ranking.

So I realize that like me you think that no-one should be forced by the
system to express any preference she doesn't have while being allowed to
express all preferences she does have.

Now, this is again a situation where I don't understand why everyone on
the list always just talks about quite a special type of preference
relations, namely rankings. Why don't they finally realize that rankings
do not satisfy the above requirement? This is even harder to understand
when examples of such preferences are so easy to construct: Prefering A
to B and C to D doesn't imply at all prefering either A to D or C to B.
But when I express A>B and C>D in ranking, I must also express either
A>D or C>B also. These preferences just don't fit into any ranking.
Instead, they build a *partial* ordering whose diagram looks like this:

	A   C
	|   |
	B   D

There is even enough evidence that people sometimes have *cyclic*
preferences when applying more than one criterion for comparing the
options. And I definitely think that those people should be allowed to
express these preferences, too. Example: Anna should be allowed to
express A>B, B>C, and C>A, meaning that when the final choice is between
A and B she would like to have A, when it's between B and C she would
like to have B, and when it's between C and A she would like to have A.

Most of our favourite election methods can easily cope with general
preference relations instead of just rankings when reformulated in the
right way! So why don't we finally stop talking on rankings and start
talking about pairwise preferences?

Sincerely, Jobst