Monday, May 28, 2018

Newspoll adjustment

The latest Newspoll estimated the Coalition would win 48 per cent, and Labor 52 per cent of the two-party preferred (TPP) vote share if an election was held now. Not a surprising result, and consistent with many other polls over recent months.

However it prompted me to do something about my Bayesian Hidden Markov Model. As many of you would know: in December 2017, Newspoll changed the way it allocates preferences in respect of the One Nation Party (see Poll Buldger, and Kevin Bonham). My Bayesian model assumes each polling house has an unchanging methodology. Clearly, we now know this is not the case. Newspoll has increased its preference flow from One Nation primary votes to the Coalition TPP vote share by an undisclosed amount. As a result, the Coalition TPP has improved by around one percentage point on average in the Newspoll series since December 2017. The easy model fix is to treat these later Newspoll results as a separate series.

Let's start today's TPP charts with what they would have been had I not changed the treatment of Newspoll within the model. The thing to note here is that the overall increase from low-point to now is 0.9 percentage points.

Now, let's look at the results when we treat the post 1 December 2017 Newspolls as being a different series. The increase from the low-point to now is little changed in the model. Treating Newspoll as a single series (with the same data) the increase from the low-point was 0.9 percentage points, and treating Newspoll as two series, this increase is 0.8 percentage points.

The new treatment also changes the vertical positioning of the lines, but this is largely an artifact of the sum to zero constraint on house effects across the houses.

More interestingly, the new treatment reveals a change in the estimated pro-Coalition bias for Newspoll of (almost exactly) one percentage point (being the difference between -0.8295 and +0.1712).

What this suggests is that today's result might have been closer to 47 to 53, Coalition to Labor, if Newspoll had not changed its preference flow methodology. I happen to think on balance that Newspoll's newer methodology is the more accurate. It certainly reflects recent state government election preference flows. But given One Nation's history I have one (huge) caveat. If, come the election, One Nation decides to preference against the Coalition on its how to vote cards, then I suspect the recent state elections will not mean a thing.

Usual acknowledgement: the data for this analysis comes from Wikipedia.

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