Saturday, December 15, 2012

Lessons on pooling polls from the 2010 federal election

With thanks to the reader who sent me the Galaxy data in the lead-up to the 2010 election, I have re-run the earlier analysis of house effects and the likely pathway for population voting intention.

That final Galaxy poll (52-48 for Labor) is illustrative of the challenge when analysing individual polling statistics immediately prior to an election. The most likely population voting intention pathway (the red line in the above chart) is within the margin of error of (+/-) 3 per cent for the final Galaxy poll. The Galaxy poll was statistically accurate. You cannot ask for more from an individual poll.

However, as we know, the final outcome of 50.12 to 49.88 per cent in Labor's favour produced a hung parliament. If the outcome had been at the centre of the distribution implied in final Galaxy poll, it would have produced a sizable Labor win. The irony is that Galaxy's house effect over the period was slightly pro-Coalition.

If we re-run the above analysis (1) without anchoring the end-point to the election result and (2) if we introduce the constraint that the house effects will sum to zero we get the following plots.

These plots remind us that pooling the polls does not automatically result in an unbiased estimate of the population voting intention. There is no guarantee that house effects will cancel each other out. In this case, the pooled polls were out by 1 percentage point. In 2010 it turned out to be the difference between a hung parliament and a comfortable win for Labor.

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