[EM] Party lists and candidate multiwinner elections

Toby Pereira tdp201b at yahoo.co.uk
Mon Oct 20 09:29:02 PDT 2014

From: Kristofer Munsterhjelm <km_elmet at t-online.de>

>So with that in mind, let me alter the example.
>You have a set of ballots that rank (or rate, or approve, etc) n 
>candidates. First, consider the candidates to be parties and run a party 
>list election with, say, 500 seats. k parties will be elected. Then run 
>a k-seat multiwinner election using the same ballots, but let the 
>candidates be individuals.

>Would there then be any situation where the set of parties that got at 
>least one seat in the assembly would ideally differ from the set of 
>candidates that got elected in the k-seat multiwinner election? If so, 
>when and why?

In the case with 500 seats, there will be far less rounding effects, and parties will be able to win seats very accurately according to their proportion of the support.

But when you just have k seats (say, 10), the top ten won't all have exactly 10% of the support. Someone might have 50%, someone might have 2%. In this case, depending on the voting system used, the second choices of the voters of the most popular candidates will become more significant. There might be a candidate who has no first choices but is the second choice for all of the 50% who support the most popular candidate. So in this case, they are very likely to be elected. But in the 500-seat example, they might not be, particularly if we are using a ranked system.

So yes, I'd say there will be cases where they would give different results. If I understand correctly, in the k-seat example, it's effectively a party election but with each party only fielding one candidate. Since there is likely to be a lot of "over-support", which won't exist in the 500-seat example, there would and should be different results in some scenarios.
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