Nicholas (nwhyte) wrote,

STV point

An occasional correspondent has asked me about transfers in STV elections, and why plumpers' votes don't get taken into account when surpluses are dealt with. It's an important point, if a little technical, so I expand on this below.

If the quota is 10, and I get 16 first preference votes, but only 8 of those votes have lower preferences for other candidates in the race, then each of those transferring votes goes on at a value of 0.75 (6, being the surplus, divided by 8, being the number of votes being transferred).

Some people find it odd that the 6 votes of the surplus are not split by the ratio of plumpers to non-plumpers. The argument is, if the surplus is 6, and I got 16 votes, then each of those should be worth 0.375 on later counts, including the ones that don't transfer anywhere.

This argument is wrong. On that basis, each of the plumpers gets not only full value for the first preference vote cast for me, but also 0.375 of an extra vote when the surplus is transferred; while those who did cast lower preferences get only the 0.375 transferred votes and 0.25 of a vote for me.

The system as it usually operates give the plumpers full value for their vote for me, and then takes them out of tally, along with an appropriate fraction of the transferable votes cast for me. That's why the value of each transferred vote is calculated as the surplus divided by the number of all transferring votes, not the total number of first preferences.

There's a real life example of this in the 2007 election in South Antrim. Mitchell McLaughlin got 6313 first preferences; the quota was 5454, so his surplus was 859. But of those 6313 votes, 751 had no usable lower preference (either plumping for McLaughlin or, rather less likely, having a second preference for Willie McCrea who was also elected on the first count). So the 5562 transferring votes had a value of 0.15 (= 859/5562 rounded off) rather than 0.13 (= 859/6313 rounded off).

It makes sense really!

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