Saturday, February 9, 2013

A different look at house effects over time ...

House effects are the systemic biases that consistently affect a pollster's published estimate of the population voting intention. In the Bayesian model, we assume they a constant over time.

In this analysis, using localised regressions (LOESS), we can see the slow drift in Essential's population estimate compared with the other three polling houses (that poll on a fairly regular basis). This is not the first time we have reflected on Essential's drift. But it is the first time we have done it with LOESS.

Another factor I have been thinking about is non-linearity in the response of polling houses to changes in the population voting intention. If you consider the 365-day-LOESS-span chart, it can be argued that over the past 18 months Morgan's face to face poll has been more responsive to shifts in public voting intention for Labor than Essential (and perhaps Newspoll). What is less clear to me is whether some polling houses are over-responsive and/or others are under-responsive.

Some key dates to think about in considering this period include:
  • Abbott replaced Turnbull as Opposition Leader - 1 December 2009
  • Rudd announced that the Government would delay implementing an emissions trading scheme - 4 May 2009
  • Gillard replaced Rudd as Prime Minister - 24 June 2010
  • 2010 election - 21 August 2010

And the combined LOESS charts for the same periods ... the drift in Essential's house effect means that the combined LOESS leans more towards Labor at the beginning of each chart than it does at the end of that chart.

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