[EM] STV - the transferrable part is OK (fair), the sequential round elimination is not

[Juho]

I wouldn't be as strict as saying that Droop proportionality is an  
absolute requirement. I'd be happy to classify all methods that  
approximate the principle of x% of votes means x% of seats as  
"acceptable PR".

Note that even if some method strictly follows e.g. Droop  
proportionality there may be other factors that distort the picture.  
It is for example typical that the size of electoral districts causes  
bigger deviation from proportionality than the method that is used  
within each district. In the extreme case single member districts may  
give disproportional power to few (e.g. two) parties (even if the  
actual method would be proportional (like plurality in a way is for  
single member districts :-)). Also e.g. 10 districts of 10 seats each  
typically means considerable bias in proportionality in favour of the  
large parties.

If the votes (and proportionality) are counted at national level that  
fixes the (district fragmentation related) problem. STV is at its best  
in small districts with small number of candidates and seats, so it  
typically leaves some space to distortion in proportionality as caused  
by the district structure. List based methods have also similar  
problems but in them it is easier to have the whole country as one  
district (=> better proportionality but weaker local representation  
(and as a result weaker "regional proportionality")), or they can be  
easily extended to count the "political proportionality" at national  
level but still allocate the seats in the districts (and thereby  
maintain also "regional proportionality" and more local representation).

My point thus is that proportionality should be observed at the  
"national level", taking into account also factors like districts and  
number of available candidates and parties, cutoffs, restrictions in  
nomination etc.

Juho



On Nov 1, 2009, at 9:03 PM, Jonathan Lundell wrote:

> On Oct 31, 2009, at 7:29 AM, Kathy Dopp wrote:
>
>> 3. STV does *not* achieve proportional representation at all unless
>> there is no vote splitting and just the right number of candidates  
>> run
>> who support each group's interests. I.e. the success of methods like
>> STV to achieve proportional representation rest in the unlikely
>> assumption that just the right proportion of candidates run (or more
>> precisely an equal proportion of candidates run) in proportion to the
>> number of voters in each separate group.  This is just simple
>> mathematical fact.
>
> STV satisfies the Droop Proportionality Criterion. Any competing  
> proposal for a proportional system must accomplish at least that, it  
> seems to me, to be taken seriously.----
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