Was YouGov the winner of the 2016-19 polling season?

I have been wondering whether the pollsters have been off the mark for some years, or whether this is something that emerged recently (say, since Morrison's appointment as Prime Minister or since Christmas 2018). Today's exploration suggests the former: The pollsters have been off the mark for a number of years this electoral cycle.

Back in June 2017, international pollster YouGov appeared on the Australian polling scene with what looked like a fairly implausible set of poll results. The series was noisy, and well to the right of the other polling houses at the time. Back then, most pundits dismissed YouGov as a quaint curiosity.

Date	Firm	Primary %					TPP %
		L/NP	ALP	GRN	ONP	OTH	L/NP	ALP
7-10 Dec 2017	YouGov	34	35	11	8	13	50	50
23-27 Nov 2017	YouGov	32	32	10	11	16	47	53
14 Nov 2017	YouGov	31	34	11	11	14	48	52
14-18 Sep 2017	YouGov	34	35	11	9	11	50	50
31 Aug - 4 Sep 2017	YouGov	34	32	12	9	13	50	50
17-21 Aug 2017	YouGov	34	33	10	10	13	51	49
20-24 Jul 2017	YouGov	36	33	10	8	13	50	50
6-11 Jul 2017	YouGov	36	33	12	7	12	52	48
22-27 Jun 2017	YouGov	33	34	12	7	14	49	51

The 2017 YouGov series was short-lived. In December 2017, YouGov acquired Galaxy, which had acquired Newspoll in May 2015. YouGov ceased publishing poll results under its own brand. Newspoll continued without noticeable change. By the time of the 2019 election, these nine YouGov polls from 2017 had been long forgotten.

Today's thought experiment: What if those nine YouGov polls were correct (on average)? I can answer this question by changing the Bayesian aggregation model so that is centred on the YouGov polls, rather assuming the house affects across a core-set of pollsters sum to zero. Making this change yields a final poll aggregate of 51.3 per cent for the Coalition. This would have been remarkably prescient of the final 2019 election outcome (51.5 per cent).

The house effects in this model are as follows.

And if we adjust the poll results for the median house effects identified in the previous chart, we get a series like this.

YouGov is a reliable international polling house. It gets a B-grade from FiveThirtyEight. When it entered the Australian market in 2017, YouGov produced poll results that were on average up to 3 percentage points to the right of the other pollsters. The election in 2019 also produced a result that was around 3 percentage points to the right of the pollsters. That a respected international pollster can enter the Australian market and produce this result in 2017, suggests our regular Australian pollsters may have been missing the mark for quite some time.

Note: as usual, the above poll results are sourced from Wikipedia.