[EM] Explaining PR-STV

[Raph Frank]

On Fri, Aug 28, 2009 at 3:30 PM, Jan Kok<jan.kok.5y at gmail.com> wrote:
> This is a very nice, clear explanation or PR-STV.

Thanks.  My aim was to get down to the reasons for each of the rules.

PR-STV is an attempt to solve the issues with PR-SNTV.

> I would suggest
> mentioning that the quota is commonly set at greater than 1/(N+1)
> times number of valid votes. Thus, with 5 seats and 600 votes, a
> candidate who gets more than 100 votes is guaranteed a seat.

I was aiming for zero maths formulas.

I am not sure how much it really adds.  The important point is that
the quota is the number of votes you need to be sure of being
guaranteed to be 5th or better.  The exact way of calculating it is
not important.

Anyone interested in the maths would be able to work it out pretty quickly from

"If 5 candidates had a quota of votes, then even if all the rest of
the votes go to one of the other candidate, he would have less than
the quota."

Also, the way I define it, the Droop quota is the only one which meets
the condition.

> I'm not convinced that PR can lead to "instability." Isn't that more a
> property of the parliamentary system? After all, in the US we can have
> congress be at 50/50 Dems/Republicans, where just one defection can
> swing control to the other side, yet our government doesn't seem all
> that "unstable".

Plurality will take a 55 to 45 split in support and magnify that into
say a 65 to 35 split in seats.

However, PR with lots of parties is less likely to swing to extremes.
If a centerist party holds balance of power, then if they shift
support the resulting government will still probably be generally
centerist (just leaning in the other direction).

I think also the point is that if a small party ends up with balance
of power, that creates an incentive for new parties (and independents)
to arise.