Summer polling?

From RadioNZ this morning, Ben Thomas on the latest polling results

I think there’s a bit of a caution. Both of these polls came very shortly after the summer break, and when you look at polls over a year, the government of the day does best when people feel best about themselves. When do people feel best about themselves? Well, it’s when they’re on holiday, when they’re looking at barbeques… when the sun is shining.

That’s certainly reasonable. You could also think of other regions summer might be different, too. For example, with schools on holiday, you might get a different range of people being at home and answering the phone. In any case I wanted to see how much it shows up in the published opinion polls.

Peter Ellis has collected polling data from September 2002 right through to the last election, so I used that. Now, popularity of the government of the day has varied over this time, so I subtracted off a party difference, differences between polling companies, and a long-term time trend. Here’s the left-over variation when the long-term trend was averaged over about 5 years

and here’s when the long-term trend was over more like one year

The purple line estimates the seasonal variation, from a linear regression model.  Polls do seem more favorable to the incumbent during summer, but it’s a very small effect. The summer to winter difference is about half a percentage point, and there’s only fairly weak evidence that it’s in that direction rather than some other direction.

Here’s the same information, but wrapped around a yearly circle, with the points coloured according to whether Labour or National was in government at the time.  The black line is a circle corresponding to zero on the graphs above; the purple line shows the seasonal difference. If you look closely, you can see the purple sticks out to the sides more than the black: summer is more positive, winter is more negative. The labels are at January 1 for summer and then regularly spaced through the year.

What you do see clearly in this format is that people don’t do many polls around the new year. But the seasonal difference in results (for party intention, in publicly-released opinion polls) seems pretty small.