September 6, 2017

Threshold and discards

There have been a few discussions on Twitter about what happens to votes for parties who don’t make the threshold of 5% or one electorate. I’m going to try to make it clearer than is feasible in 140 characters, but without mentioning quotients.

If you voted for a party that gets less than 5% of the party vote and does not win any electorate, your party vote is not used in determining the list seats.  It doesn’t get reassigned, reweighted, or re-anything. It just isn’t used — exactly as if those people hadn’t cast a Party vote. (Electoral Act, section 191 (4)). Last time, about 150,000 votes were set aside at this point.

The votes for the parties that are left (2.3 million, last time) are now used to allocate 120 seats.  Complicated procedures are used to work out a number of votes per seat, call it N.  The total for each party is divided by N, and rounded to the nearest whole  number, so you need at least ½N to get one seat, 1½N to get two seats and so on. (That’s not how the Electoral Act describes it; this is the equivalent ‘Webster’ method rather than the ‘Sainte-Laguë’ method).

So what are the implications?

I don’t like the term ‘wasted’ vote — if you’re voting for, say, Ban 1080 or Aotearoa Legalise Cannabis, it’s presumably not in the expectation of getting representation in Parliament, but more as a way of making your views known.  However, if your intent is to increase representation in Parliament of people whose views you support, this is the basic guideline

  • If the opinion polls show a party is nowhere near the 5%/one electorate threshold, the expected impact of increased votes for that party  on the composition of Parliament is very small (compared to a major party)
  • If the opinion polls show a party is close to the 5% threshold (in either direction) and isn’t certain to get an electorate, the expected impact of increased votes for that party  on the composition of Parliament is relatively large (compared to a major party)
  • If a party is reasonably certain to get an electorate or to get over the 5% threshold, he expected impact of increased votes for that party  on the composition of Parliament is about the same as for a major party.
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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

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