The MMP threshold

MMP is, in many ways, a beautiful voting system.  As implemented in New Zealand it’s got one feature that complicates voting slightly and complicates forecasting a lot: the threshold.

In TVNZ’s Colmar Brunton poll, the Greens got 4.7% of the vote. The threshold is 5%. Getting 4.7% of the vote in an election would mean you don’t get any seats.   The margin of error that TVNZ were stating was +/- 3%, so based just on that, the Greens were basically 50:50 on whether they make the threshold.

At that level it also potentially makes a big differences how you treat the undecided voters, who made up 16% of responses. The person who pointed this out to me thought that the 16% had been left as a separate group, which you might easily think from the TVNZ web post on the poll.  But if you do the arithmetic, the parties’ quoted percentages add up to 100 (give or take rounding error), so the percentages were of those who expressed a preference.  Only 4.1% of the respondents actually said “Green”.

Normally, you don’t have to worry about this because tiny changes in preferences will only produce small changes, if any, in the number of seats. But going from 4.99% to 5.00% takes you from no seats to several (I think six) seats. Predicting the make-up of Parliament gets hard when there are parties close to the threshold.

Given the sensitivity of the results to small changes, I think the website (if not the actual news segment) should be more explicit about how undecided voters are handled in the seat projection.  And making statements about whether a party is in or out of Parliament should have a bit more explicit uncertainty when it could be wrong by six seats with minute changes in voting.

On the TVNZ website, the report says “ACT, assuming it wins an electoral seat, would pull in three MPs with its 2.2% support”, and if National’s gift of Epsom to David Seymour is enough of an uncertainty to require an explicit caveat, so is being a few voters per thousand away from the threshold.