[EM] Explaining PR-STV

[Jonathan Lundell]

On Aug 28, 2009, at 10:40 PM, Kristofer Munsterhjelm wrote:

> Markus Schulze wrote:
>> Hallo,
>> I would explain proportional representaion
>> by the single transferable vote as follows:
>> 1) Each voter gets a complete list of all
>> candidates and ranks these candidates in
>> order of preference.
>> 2) Suppose M is the number of seats and
>> V is the number of votes. If there is a
>> set of X candidates such that strictly more
>> than (Y*V)/(M+1) votes strictly prefer
>> each candidate of this set to each candidate
>> outside this set, then at least min{X,Y}
>> candidates of this set must be elected.
>
> Isn't that true of all methods that obey the Droop proportionality  
> criterion (including your own STV variant, which does not transfer  
> any ballots)?

It's also true of QPQ.

DPC is certainly crucial to any PR method, but it doesn't capture the  
difference between STV and other methods that satisfy DPC. Nor does it  
address the question of election profiles that don't have M solid  
coalitions.