[EM] How to combine list and candidate ranking based proportionality?

[Kristofer Munsterhjelm]

Aaron Armitage wrote:
> I've considered the question myself, although I've never described my
> ideas publicly. Now's as good an opportunity as any.

[snip]

> The first way of adding lists to STV is simple: you list your candidates,
> and last you put a list, which fills out the rest of your preferences
> according to the predefined order of the list. I suppose you could include
> more than one list, or a list then a candidate, but that would be
> pointless because your vote would be used up.
> 
> A simple example: The Yellow list is A>B>C>D>E, and
> the Brown list is F>G>H>I>J. If you vote B>I>Yellow, it counts as
> B>I>A>C>D.

An advantage of this is that you can vote "cross-list". I think the 
ballot design would be challenging, though; in order for the voter to be 
able to vote cross-list, the preference ballot would have to reference 
all the candidates on all the lists, so that the voter can vote B ( 
which is on the Yellow list) > I (which is on the Brown) > lists.

> The second way is more sophisticated, and much more complex to count. The
> vote itself needn't be any more complex (you could always just vote a list
> and leave it at that), but it can be, and depending the layout of the
> ballot it may look more visually confusing. It will really need to be done
> with a touch-screen, preferably using a drag-and-drop interface. A paper
> ballot should be printed out and kept as a check (or perhaps the paper
> count should be all there is, the computer interface being simply the
> means of generating the paper ballots).
> 
> Instead of using regular STV, the second way uses CPO-STV. Since it's a
> Condorcet method, it allows tie votes which amount to voting present in
> the choice between them. Or, in the case of CPO-STV, between two outcomes
> which differ only in electing one or another of tied candidates. The lists
> are unordered and instead of representing a completed ordering filled in
> at the end of the ranking, they are a tie between all list members. The
> party lists will probably be mutually exclusive, but there's no reason not
> to have other lists which overlap the party lists and each other. If a
> candidate appears in two lists which are ranked on the same ballot, he
> takes the higher of the two positions, but if he is ranked individually,
> as a candidate, he takes that rank regardless of how any of his lists
> might be ranked.
> 
> So, going back to the previous example, say you vote
> B>Vowels>Yellow>Brown>Purple>F>White. This becomes:
> 
> B > (A,E,I,O,U,Y) > (C,D) > (G,H,J) > (K,L,M,N) > F > (P,Q,R,S,T)

You could also use Schulze STV -- if the lack of polynomial runtime 
isn't a problem. For lists, you'd have many candidates, so it might.

For the sake of completion, I can also mention that in my monotone 
Webster-based multiwinner method, the "rank within a list" and "expand 
list to preference ballot" methods would have the same outcome. This is 
because the method has the property that if everybody votes preferences 
in a disjoint manner - i.e. no cross-list voting - then each party is 
given a number of seats equal to how many they would have got if voters 
voted for the party list and the seats were apportioned by Webster's method.
Thus that method would be the closest to bridging lists and preference 
ballots, because voting in a "list fashion" gives the same result as 
actual party list voting.
Unfortunately, that method is not polytime either, and the single-winner 
results are not very good. It's more a proof of concept than anything -- 
maybe it's possible to retain that "list similarity" property in better 
multiwinner methods.