Tuesday, December 4, 2012

The weekly Bayesian poll aggregation: 48-52

I am now in a position to publish a weekly aggregation of the national opinion polls on voting intention. The headline message from the first aggregation: following an improvement in Labor's fortunes between July and mid October, national two-party preferred (TPP) voting intention has flat-lined since mid-October. If we assume that all of the house effects sum to zero over time, then the current outcome is 48.2 to 51.8 in the Coalition's favour.

At the moment, I am aggregating five separate polls: Essential, Morgan (treating the face-to-face and phone polls separately), Newspoll and Nielsen. The polls appear in the aggregation on the mid-point date of the polling period. Because Essential's weekly polls appear twice in their weekly report (as a fortnightly aggregation), I only use every second Essential report (beginning with the most recent).

This aggregation assumes that the systemic biases across the five polling streams sum to zero. Because this is an unrealistic assumption, I also produce a chart on the relative house effects of the polling houses for the period under analysis. The distance from the house median and the zero line indicates the biases applied to or subtracted from polling houses in the above chart. (Note: These will move around over time.) You can then decide whether the estimate of the national population voting intention in the first chart needs to be adjusted up or down a touch.

As a rough indication, this outcome would see the following probabilities for the minimum number of seats won.

This is not the end point in my opinion poll analytical efforts. I have started work on a Bayesian state-space model for Australia's five most populous states. This should allow for a more accurate outcome prediction on a state-by-state basis.

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