Wednesday, June 10, 2015

Where to locate the Bayesian analysis

There are at least three options for anchoring the Bayesian analysis. I could:

  • use the most recent election result and anchor the state of the hidden markov model for that day to the election result;
  • decide that one or more pollsters on average is/are absolutely correct (that is to say individually or collectively they have a house effect of zero), or at least has a known constant house effect; or
  • locate the poll estimate such that we assume the polling industry collectively is unbiased (that is to say, the sum of the house effects is zero).

Recently, I have been using the last approach. But others would argue that the first approach is most accurate. Writing here, Simon Jackman noted that election anchored models did better in 2010 than  sum-to-zero models. It looks like a similar outcome in the 2013 election.

In the 2013 election, I ran a number of different models. One model simply used the polling data from Newspoll and Neilsen, the two pollsters I judged the most accurate in the past. In effect, this was the second approach I outlined above. It ended up being very close to the actual result in 2013. However, I wont have that option in 2016 with the churn we are seeing in the polling industry at the moment.

The TPP-HMM line in the next chart (the fatter, brown line) is the sum-to-zero result for all houses excluding Essential. In the eight months prior to the 2013 election, Essential is largely below this line. Since the 2013 election, its poll estimate has been above the line on average. For those who use Essential in their analysis, this repositioning, whether by chance or some change in methodology, will impact on their anchored aggregation, which assumes no change in practice or house effect on average.

The next two charts are the sum-to-zero TPP chart, followed by the election anchored TPP chart for the period since 1 January 2013. In these charts, I have only used the Morgan multi-mode polls. I have not used Morgan phone nor face-to-face polls. The difference between the two charts is half a percentage point, with the 2013 election anchored chart being more favorable to the Coalition.

In the next chart we take the medians from the previous charts. We can see that these analyses are very similar (if not the same), except they are located half a percentage point apart on the y axis.

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