Why you can’t predict Epsom from polls
The Herald’s poll aggregator had a bit of a breakdown over the Epsom electorate yesterday, suggesting that Labour had a chance of winning.
Polling data (and this isn’t something a statistician likes saying) is essentially useless when it comes to Epsom, because neither side benefits from getting their own supporters’ votes. National supporters are a clear majority in the electorate. If they do their tactical voting thing properly and vote for ACT’s David Seymour, he will win. If they do the tactical voting thing badly enough, and the Labour and Green voters do it much better, National’s Paul Goldsmith will win.
Opinion polls over the whole country don’t tell you about tactical voting strategies in Epsom. Even opinion polls in Epsom would have to be carefully worded, and you’d have to be less confident in the results.
There isn’t anywhere else quite like Epsom. There are other electorates that matter and are hard to predict — such as Te Tai Tokerau, where polling information on Hone Harawira’s popularity is sparse — but in those electorates the polls are at least asking the right question.
Peter Ellis’s poll aggregator just punts on this question: the probability of ACT winning Epsom is set at an arbitrary 80%, and he gives you an app that lets you play with the settings. I think that’s the right approach.
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 »