[EM] British Colombia considering change to STV

Dan Bishop danbishop04 at gmail.com
Tue May 5 17:24:10 PDT 2009

```Raph Frank wrote:
> On Tue, May 5, 2009 at 7:11 PM, Juho Laatu <juho4880 at yahoo.co.uk> wrote:
>
>> Why only fraction of the vote in the
>> election case? Doesn't a vote to a
>> party mean that any candidate of the
>> party may use it at full strength?
>> Naturally once someone uses it it is
>> not available to others at full
>> strength anymore.
>>
>
> It is the standard proposal.
>
> A=B>C
>
> gets effectively converted into
>
> 0.5: A>B>C
> 0.5: B>A>C
>
> IIRC, the reason is that it means that if you vote
>
> A>B>C
>
> and
>
> A=B>C
>
> and A and B are elected, you want the same percentage of your vote to pass to C.
>
>
The one thing you haven't mentioned is surpluses.  The
symmetric-completion-compatible way of dealing with them is weight the
ballot by the average of the retention fraction for the top-ranked
candidates.  For example, given the ballots:

4: A>B
3: A=B
5: C=D

and a quota of 4 votes, we'd have

A elected with 5.5 votes (excess of 1.5) -> retention fraction = 1.5/5.5
= 3/11
B, C, D not met quota -> retention fraction = 1

So the A=B ballots would be weighted by (3/11+1)/2 = 7/11, so the
re-weighted ballots would be:

4*3/11: (A>)B
3*7/11: B
5: C=D

3: B
5: C=D

```