Reading the Political Tea Leaves

Despite mentioning tea in the title, this post is about opinion polls, not the ACT of drinking tea.

There are a number of sites that maintain graphs of New Zealand political polls, notably Rob Salmond at Pundit and, curiously, Wikipedia. Wikipedia post their data in tabular form, and provide the code for R underlying the creation of their graphs, allowing anyone with certain degree of geek cred to have a hack (ie, me). Yesterday, someone on wikipedia requested an updated version focussing only on more recent polls, which I had a go at, including making the fit line a bit more sensitive to change.

Changing the sensitivity of the fit line, makes it seem like there more movement than there has been in a while, and this graph was subsequently featured on the DimPost and then the Listener.

The next question was about whether the different polling parties differentially contribute to such a rolling poll. Their sample sizes are pretty uniform, at around 800. Roy Morgan contribute over half (68 of 121) with their poll regularly conducted over several days. 3 News Reid Research have fewer polls (16) but all conducted on a single day.

But how do the different parties fare in the polls (in order of polling).

Firstly, you can really get a sense from this graphic how much more regular Roy Morgan are, and their estimates are pretty consistently low, relative to the other pollsters. 3 News Reid Research is fairly consistently high. However, in the latest few polls, they have had some lower numbers for National.

3 News Reid Research is consistently lower on Labour, which combined with the above, suggests they favour National relative to the other pollsters. The Herald Digipoll has higher values for Labour most of the time.

The Herald Digipoll and ONE News Colmar Brunton are consistently lower for the Greens than other pollsters. However, in the most recnet polls, Herald Digipoll and ONE News Colmar Brunton both show much higher numbers than usual for the Greens.


  • There is definitely variation attributable to the pollsters.
  • It does seem like there is some change, at least with the polls. Whether this translates to anything meaningful for next Saturday, who knows. I plan to add ACT and NZ First later for completeness.

About James

I'm a scientist. Sometimes I get distracted.
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