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.

A month at the betting market

I have been following the Australian Federal Election betting market for a month now. Not a lot has happened in that month. Across the four houses I regularly sample, punters think that Labor has a 63 per cent chance of winning the next election and the Coalition a 37 per chance. On 15 April, when I started collecting odds from these four houses, it was 65 and 35 per cent.

May poll aggregate update

Another month has passed, so it is time to update the poll aggregate. Let's start with a list of the recent poll results, sourced from the Wikipedia page on the next federal election.

	MidDate	Firm	L/NP	ALP	GRN	ONP	OTH	TPP L/NP	TPP ALP
1	2018-04-30	ReachTEL	36.0	35.0	11.0	6.0	12.0	48.0	52.0
2	2018-04-20	Essential	37.0	36.0	11.0	8.0	8.0	47.0	53.0
3	2018-04-22	Newspoll	38.0	37.0	9.0	7.0	9.0	49.0	51.0
4	2018-04-06	Essential	38.0	37.0	10.0	7.0	8.0	47.0	53.0
5	2018-04-06	Newspoll	38.0	37.0	10.0	7.0	8.0	48.0	52.0
6	2018-04-04	Ipsos	36.0	34.0	12.0	NaN	18.0	48.0	52.0

Tossing these results into the two-party preferred aggregation reveals.

This can be compared with my simplified aggregation models using Henderson moving averages (HMA) and Locally Weighted Scatter-plot Smoothing (LOWESS). Both of these models can do strange things at the end points. Given the data, I suspect they are a touch over-enthusiastic for the Coalition come the end of April 2018.

Collectively we can see that the Coalition's fortunes continued their rise through April. Nonetheless, if an election was held now, the most likely outcome would be a sizable Labor victory.

Moving to the primary voting intention results.

Collectively, the implied two-party preferred (TPP) results for the Coalition follow. Unlike the TPP charts above, calculating the TPP results directly from the primary vote sees a stagnating TPP. I suspect the issue here is partly affected by Newspoll's change in methodology for attributing preference flows for One Nation. While I can understand Newspoll's decision to increase the One Nation to Coalition preference flows (based on preference flows at the recent Qld and WA state elections), the changed methodology is inconsistent with my models. I will need to think about this some more.

Speaking of One Nation, there has been a slow decline in its vote share over recent months.