New Zealand general election forecasts - changelog

Significant changes to the New Zealand election forecasts web page

All dates and times are New Zealand time.

  • 16 September 2017 Add first published poll by Horizon (Model A only, as Model B needs two polls per pollster minimum). Also adds a minor tweak to Model B to allow for the Reid Research house effect to change in 2017, when their method changed (a quarter of the sample is now online).
  • 15 September 2017 Add latest Roy Morgan poll, no big change to substance of forecasts.
  • 14 September 2017 Add the One News Colmar Brunton poll, more positive for Labour than was the latest Reid.
  • 12 September 2017 Add in the Newshub Reid Research poll, which again is less positive for Labour
  • 10 September 2017 Add to both models in three Listener - Bauer Media Insights polls that only recently were added to Wikipedia, and to the GAM an additional small SSI poll. Models have now converged on very low chance of National-led coalition.
  • 7 September 2017 Add a Colmar Brunton poll, continuing to be more positive for the Labour-led coalition. Also took the call to shuffle around the website to indicate that the combined model of the two is now the preferred one. Model A doesn’t react fast enough to changes in the atmosphere.
  • 4 September 2017 Add the latest Newshub Reid Research poll, which is less positive for Labour than the Colmar Brunton
  • 31 August 2017 Add the dramatic Colmar Brunton poll with Labour passing National for first time in years. Net result is big reduction in the chance of current Nat-led coalition winning.
  • 21 August 2017 Update subsequent to Peter Dunne pulling out of Ohariu electorate. Net result is about a 2.5 percentage point drop for the chance of current Nat-led coalition winning.
  • 20 August 2017 Found the problem with Model B, which turned out to be a data problem tracking back to Wikipedia, not caused by the volatility after all. Recent polls have had a big downwards impact on the chances of a National-led coalition without NZ First in Model B. This is partly the revision downwards of the Ohariu chances, and partly just getting closer to the election date there is less chance to pick up and a convergence of probabilities to the likely result.
  • 19 August 2017 Updated Model A with an extra poll (Roy Morgan); Model B still having problems converging so is now two polls behind. Shiny web app (“choose your own coalition”) is also two polls out of date at this point.
  • 18 August 2017 For Model A, add an extra poll (Colmar Brunton); and shift down the assumption of Ohariu going to Peter Dunne to 0.3 (from 0.6), based on the only electorate-specific poll available. Net impact is a bit of an increase in National’s chances; the interpretation of this is the model picks up recent negative polls for both the Greens and Labour but Labour’s recent positive ones aren’t enough to pull them up again. On the other hand, the volatility is clearly a bit much for the state space model and it doesn’t converge to sensible values. So Model B and the Combined model are not updated at this stage.
  • 9 August 2017 Add an extra poll (Newshub Reid Research). Not much change to the main Model A, but the more volatile Model B has now converged with it.
  • 1 August 2017 Add an extra poll (Newshub Reid Research). Impact of recent changes on the chance of a National coalition (my headline prediction are basically unchanged for the main model, Model A; the more volatile Model B is perhaps converging towards Model A.
  • 31 July 2017 Add an extra poll (One News Colmar Brunton).
  • 15 July 2017 Add an extra poll (Roy Morgan). Model A:National/coalition down to 34, back where it was in mid-June (before what now looks to have been a budget bounce). Model B: National/coalition down to 51.9. State space model (Model B)seems to more volatile, which is a further reason for using it as Model B rather than the main predictions.
  • 12 July 2017 Add Model B, an alternative forecasting method drawing on a state space model. Main difference in approach is an assumption that growth and decline in individual parties’ results to date will not continue up to election day and that latent voting intention is instead a random walk. The implication of this is a much strong outlook for a National-led coalition - 63% chance instead of 36%.
  • 11 July 2017 Add an extra poll (Colmar Brunton). National or National led coalition back down to 36%.
  • 24 June 2017 Add an extra poll (Roy Morgan). National or National led coalition continues to nudge up, now to 38%.
  • 17 June 2017 Add an extra poll. Probability of National or National-led coalition continues to nudge up, now reaching 34%.
  • 7 June 2017 Add an extra poll. Makes National and National-led coalition slightly more favourable than previously, but not really a material change.
  • 28 May 2017 Add an extra poll. Makes no material change to any of the predictions.
  • 30 April 2017 Another change to the estimation of house effects (bias by polling firm - party combination). I’ve now switched over to a Bayesian approach with an informative prior, which has the effect of shrinking these estimated biases towards zero compared to the previous method, which just took the average bias of the past 2-4 elections. This change means the Greens’ chances go up a bit and New Zealand First down a bit compared to previous forecasts, and the substantive overall outcome impact is that a National-led coalition ends up with a bit more predicted chance of winning than before. I also changed the lead graphic in the main summary to make it more compatible with the “build your own coalition” web tool.
  • 29 April 2017 Add an extra poll. Net impact is to very slightly decrease expected Labour vote. Most probable outcome is still about 80% chance of New Zealand First being needed for a coalition government. Also, rejigged the main page layout a bit (put all the 2014 predictions together at the end, and the 2017 prediction graphic at the top).
  • 16 April 2017 Add an interactive web app to “build your own coalition” and to explore the assumptions in individual electorate seats. Also increased the number of simulations used for the main page from 1,000 to 10,000.
  • 9 April 2017 Add more variance in from the process of estimating house effects. This adds materially to the uncertainty for New Zealand First in particular, which is indicative of the somewhat sketchy record of previous polls in estimating New Zealand First vote.
  • 9 April 2017 Add a histogram of individual party seat expectations
  • 27 March 2017 19:30 Fixed a problem where parties with extremely low predicted party vote also got extremely high variance, so a small number of simulations allocated them 20-30% of the party vote. Net impact was to reduce the importance of Mana party marginally.
  • 26 March 2017 20:10 Updated with one more opinion poll
  • 26 March 2017 18:00 Revised method of simulating the randomness of an election result on top of the predicted latent party support. Previous method was to make elections as random as an opinion poll; this added too much variation to simulations. The new method models squared error of forecasts and actual results of previous elections (2014 and before) to provide an estimate of the variance of actual election results around the mean vector of latent party support. This variance is combined with the correlation between party support from the forecast model to estimate a variance-covariance matrix, on a logit scale, which is used for the simulations of election day party vote. The impact of this change is to greatly reduce the uncertainty in the model, and hence focus the probability mass on a smaller number of likely outcomes. For the retrospective “forecast” of the 2014 results, it makes the National Party coalition by far the most likely outcome (as turned out to be the case), even from six months out. See the change to the source code.
  • 26 March 2017 10:00 Corrected error for minor parties in the retrospective 2014 forecast, which had inserted 50% support for polls which should have indicated 0%. The impact of fixing this bug was to reduce “National coalition” chances of winning the 2014 election, as seen from six months previously
  • 26 March 2017 9:00 Initial public version