Variability of polls

Opinion polls can vary markedly, between pollsters and from poll to poll. They are a snapshot indicator of public opinion with known margins of error.

Yesterday’s Roy Morgan poll sparked some hope on the left with a big increase in Labour support and an almost as big drop in National support – from the previous poll. But if you look at a number of polls through the year the spikes and dips don’t seem so dramatic.

National dropped 5 to 46% but are within the range they have been polling with Roy Morgan this year (they 2-3 polls per month, usually fortnightly):

  • 43.5% in January, 43% in March, 42.5% in April/May, 45.5% in May
  • 47% in January, 48.5% in February and March/April, 52.5% in May/June, 51% in June/July
  • This week’s poll 46%

So a move from 51% to 46% doesn’t seem out of the ordinary. Nine out of fourteen times this year National’s poll to poll movement has been more than 3%, four out of fourteen times it has been more than 5%. Fluctuation is normal.

Labour rose 6.5 but their last result of 23.5% was suspected to be an outlier. This year:

  • 30% in February, 28.5% in March, 28% in June, 23.5% in July
  • 33.5% in January, 32% in March, 30.5% in May, 29% in May/June
  • This week’s poll 30%

Labour’s moves have been less dramatic with five of fourteen being 3% or more and the only 5+ moves being down to the low of 23.5% last poll and their bounce back to a more normal range this poll.

Greens poll lower so their movements could be expected to be smaller but they have bounced around too:

  • 11% in January, 11.5% in February, 9% in May
  • 12.5% in January, 14% in March, 14.5% in April/May, 15% in July
  • This week’s poll 12%

Six of fourteen movements have been 3% or more.

NZ First has ranged from 3.5% to 6% this year, the last four results being 4, 5.5, 6, 5 so what they might get in the election is anyone’s guess. Small parties often move proportionally more leading into an election.

Internet-Mana claimed a big move rising from 1.5% to 2.5% – but since their alliance was announced they have polled at 2.5, 2.5, 1.5, 2.5 so the movement this poll could be insignificant.

Movements of support from one poll to another often make the headlines but they don’t mean much. It’s normal for results to bounce around.

A pollster might be able to suggest how much this is expected poll variability and how much might be volatile public opinion.

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