There's a lot of attention on political polls at election time, and a lot of people claiming that the polls are becoming less and less accurate. It's true that polling companies face growing challenges in getting our polls accurate, but it is something we put a lot of effort into. I thought it was worth looking objectively at the public polls over time to see whether they really are becoming less accurate.
Here are the basic tools I'm using for judging accuracy:
1) The final result for every public poll published in New Zealand between 1999 and 2011 (i.e. the last result before election day)
2) The party vote for the four largest parties on average across that time period (National, Labour, Greens and New Zealand First)
3) The difference between what the polls said and what the actual result was.
Now, I'm aware that the Alliance (1999) and ACT (1999 and 2002) have made guest appearances in the top four occasionally, but I thought it simplest to use the same parties for all five elections.
I've measured the difference between the polls and the actual result using what I term 'net error'. That is, the sum of the absolute values of the differences between the polls and the actual result for the parties concerned. If, for example, a poll picks National to get 48% and Labour to get 26%, and the actual result is National 47% Labour 27%, then the 'net error' for National and Labour is 2% (made up by 1% for National and 1% for Labour). A perfect poll would therefore have a net error of 0%.
The polls I've used are: