Misleading opinion polls

In the US, the publicly reported opinion polls overestimated Biden’s lead over Trump. RCP gave Biden a nation-wide polling-average margin of 7.2 points over Trump. The actual margin is about 3.

In NZ, before the 17 October election, polls overestimated National and underestimated Labour by significant margins.

The averages of the six polls taken in the month before the election were: National 30.9, Labour 47.2.

The averages of the three polls taken in the two weeks before the election, while advance polling booths were open, were: National 31.4, Labour 46.3. It looked like the gap was closing.

On polling day, therefore, it was reasonable to expect that Labour would finish on about 46. As Labour’s trend in the polls since its stellar heights in mid-year was downward, 45 also appeared to be a reasonable prediction.

As it turned out, predictions based on the opinion polls were significantly wrong. Labour’s final election result is 50%, National’s is only 25.6%.

The polls in the final fortnight were over-estimating National by an average of 5.8 percentage points. They were under-estimating Labour by 3.7 points. The Green and Maori parties were also under-estimated (1.1 and 0.7 points respectively).

Conclusion: opinion polls in New Zealand have a conservative bias.