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Posts Tagged ‘Fairfax Media Poll’

There has been a bit of debate on Twitter about this post at The Political Scientist, which argues that the National’s rise in the polls is merely the result of Labour voters switching to “undecided” and reducing the denominator.  There were also a few requests for an explanation, and Thomas at StatsChat has obliged here (also, I stole his title).  I have a few points I want to add or expand on.

Firstly, as an aside, many of the mistakes are the same as those made in this post on Kiwiblog which argues based on correlation coefficients that in 2011 it was mainly National voters who stayed home, not Labour voters.

Secondly, we don’t actually know the number of undecided voters.  As pointed out in comments on the StatsChat rebuttal many of the raw numbers are weighted by demographics, probability of voting, and others (whether or not they also have a cellphone?).

Thirdly, the results for the correlation coefficients are very susceptible to the number of polls.  On first read this particular table from The Political Scientist stood out:

Correlation coefficients, from The Political Scientist.

Correlation coefficients, from The Political Scientist.

The table shows the correlation coefficients with the undecided vote for four parties for all nine Fairfax polls from July 2012 to June 2014 (top), and for only eight polls, with the June 2014 poll results excluded (bottom).  You can see that the correlation coefficient for National changes from 0.7 to 0.3 with the addition of a single poll!  Obviously the results aren’t particularly robust, and that is equally as true for the other three parties as well, even if they just happened to show smaller changes in the table above.

Taking this a step further, it should be reasonably obvious that you can’t trust estimates of correlation coefficients based on a small number of data points.  When you have only two data points to work with you must get a correlation coefficient of 1 even if there is no actual correlation between the things you are measuring, because for any two given points it is possible to draw a straight line that passes through both of them (or, rephrasing, two points define a straight line).  Adding more data points will move your estimate of the correlation coefficient closer to the true value, but with a small number of polls you can never be very confident.

As another aside, always be suspicious when you see results quoted to a large number of significant figures.  There’s nothing wrong with it in principle, but it raises the question of how accurate they really are.  In this particular case, if the addition of a single poll moves National’s coefficient from 0.7 to 0.3 then there’s no point quoting more than one decimal place, if at all.

Fourthly, there seems to be confusion between different coefficients.

Thomas covers this point, the difference between correlation coefficients and regression coefficients, in paragraphs 2-3.

More intuitively though, the correlation coefficients shown in the table above between NZ First and undecided voters (0.8) is almost that same as that for Labour’s.  Does the drop in NZ First support cause the increase in undecided voters?  In the last two year the number of respondents supporting NZ First fell from 32 to 24 (see linked table below), while the number of undecided respondents went from about 110 to 230.  Would you argue that the 8 former supporters per 1000 lost by NZ First turned into 220 new undecided voters?  Of course not!

Poll results, and estimated number of respondents, from The Political Scientist.

You may argue that the real evidence is that the number of supporters lost by labour is (roughly) equal to the increase in the number of respondents who are undecided, and that correlation coefficients have nothing to do with it.  And that’s fine.  But then why bother publishing the correlation coefficients at all?

Fifthly, correlation does not imply causation (see also, xkcd).  When dealing with correlation effects you have to be very careful to avoid false causation.  Even assuming the changes aren’t just a statstical fluctuation we still can’t say whether Labour voters are really becoming undecided.  As Thomas says

You could fit the data just as well by saying that Labour voters have switched to National and National voters have switched to Undecided by the same amount — this produces the same counts, but has different political implications.

If you’re a frequentist then Thomas’ alternative explanation is just as convincing.  If you’re Beysian then now might be a good time to break out Occam’s Razor and say that you thought that Labour voters were switching to undecided anyway, so you believe the first hypothesis.  Which is fine.  But in that case was there any value in the analysis?

The only way to figure out what it really going on is to do a longitudinal study where you use the same sample of voters for each poll.

Sixthly, in their conclusion The Political Scientist says

Without taking into account the level of the undecided vote this presents a misleading picture of how many voting age New Zealanders support each party.

Of course, by limiting reporting only to those who have declared party support and are likely to vote the reporting may very well reflect what could happen in an election.

This is sort of hedging both ways.  If the point of the Fairfax poll is to gauge support and/or try and predict the results of an election “if it were held today”, then the pollsters must do something with the undecided voters.  Unless you have evidence that they break differently than for decided voters (which could be the case), it seems sensible to just ignore them when publishing the headline results.  It’s not “a very misleading picture” at all.

Bonus: check out this excellent diagram from Wikipedia showing the differences between regression coefficients and correlation coefficients.  All the graphs in the middle row (except the centre) have the same absolute correlation coefficients.

Correlation coefficients.

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The latest poll is the new Fairfax Media poll from this morning, 23 November.

The updated polling averages now have National on 51.7% +/- 0.6%, Labour on 27.8% +/- 0.6%, the Greens on 11.6% +/- 0.4%, NZF on 3.5% +/- 0.2%, ACT on 1.5% +/- 0.1% and the Maori Party on 1.6% +/- 0.1%. Changes are not statistically significant relative to the previous update.

As usual, the two graphs below summarise the polling averages for the party vote after the new poll. The horizontal axes represent the date, starting 60 days before the 2008 NZ General Election, and finishing on day of the 2011 NZ General Election (26 November 2011). The solid lines with grey error bands show the moving averages of the party vote for each party, and circles show individual polls with the vertical lines representing the raw statistical errors.

Party vote support for the eight major and minor NZ political parties

Party vote support for the eight major and minor NZ political parties as determined by moving averages of political polls. Colours correspond to National (blue), Labour (red), Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

Party vote support for the Green party

Party vote support for the Green party as determined by moving averages of political polls.

Party vote support for the five minor NZ political parties

Party vote support for the five minor NZ political parties as determined by moving averages of political polls. Colours correspond to New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

As far as the election results go, National is still predicted to win an outright majority.

As always, please check the Graphs page for further simulation results.

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This update is based on 14 new polls published between 23 October to 18 November. I won’t list them all, but they can all be found on the Opinion polling for the New Zealand general election, 2011 page on Wikipedia. Much thanks to whomever is updating that page.

The updated polling averages now have National on 51.8% +/- 0.6%, Labour on 27.8% +/- 0.6%, the Greens on 11.5% +/- 0.4%, NZF on 3.5% +/- 0.3%, ACT on 1.6% +/- 0.2% and the Maori Party on 1.6% +/- 0.1%. Relative to the last update a month ago, the polling averages show a statistically significant drop in support for National of 3% +/- 2%, and rise for the Greens of 2% +/- 1%. There has also been a rise for New Zealand First of about 2% +/- 1%.

The drop for National has them about back at their average for the last 24 months, and NZF are still about where they were six months or so ago. The Greens’ rise, however, comes on the back of a series of smaller rises going back about six months or so. The are up roughly 4% +/- 1% on from their typical 7% support through most of this electoral cycle (see graph below).

As usual, the two graphs below summarise the polling averages for the party vote after the new poll. The horizontal axes represent the date, starting 60 days before the 2008 NZ General Election, and finishing on the present day. The solid lines with grey error bands show the moving averages of the party vote for each party, and circles show individual polls with the vertical lines representing the raw statistical errors.

Party vote support for the eight major and minor NZ political parties

Party vote support for the eight major and minor NZ political parties as determined by moving averages of political polls. Colours correspond to National (blue), Labour (red), Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

Party vote support for the Green party

Party vote support for the Green party as determined by moving averages of political polls.

Party vote support for the five minor NZ political parties

Party vote support for the five minor NZ political parties as determined by moving averages of political polls. Colours correspond to New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

As far as the election results go, National is still predicted to win an outright majority, although we are starting to get to the stage where any further drops in supprt for National may start to have consequences (as long as they are polling over 51% it doesn’t really matter how well they do, from a who’s-in-government perspective). The simulation currently gives National a 99.98% chance of winning an outright majority, and a 0.02% chance of leading a National-ACT coalition.

Scenario analysis for the most recent election simulation.

Scenario analysis for the most recent election simulation. The bar graph shows the probabilities for different possible outcomes for a NZ General Election if held on the date of the current update (please see the top of this page for date.) The National Party would have a roughly 100% chance of governing alone, a roughly 0% chance of governing as leader of a National-ACT coalition, and a roughly 0% chance of governing as leader of a National-ACT-United Future coalition. There is a roughly 0% chance that the Maori Party would hold the balance of power in Parliament.

As always, please check the Graphs page for further simulation results.

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Two more polls from Roy Morgan Research and Fairfax Media have been published in the last week. Unfortunately I didn’t have time to include the two new polls from the 6PM news tonight in the calculations, but I’ll do another update in a day or two.

The updated polling averages now have National on 52.2% +/- 1.2%, Labour on 29.9% +/- 1.1%, the Greens on 9.4% +/- 0.6%, NZF on 2.4% +/- 0.5%, ACT on 1.8% +/- 0.4% and the Maori Party on 1.9% +/- 0.3%.

Relative to the last polling update National is down about 2% +/- 1%. The new numbers have National more in line with their average for the last year. The average for the Greens is up about 0.5% again, a statistically insignificant change, but continuing a consistent upward trend seen over the last six months or so.

As usual, the two graphs below summarise the polling averages for the party vote after the new poll. The horizontal axes represent the date, starting 60 days before the 2008 NZ General Election, and finishing on the present day. The solid lines with grey error bands show the moving averages of the party vote for each party, and circles show individual polls with the vertical lines representing the raw statistical errors.

Party vote support for the eight major and minor NZ political parties

Party vote support for the eight major and minor NZ political parties as determined by moving averages of political polls. Colours correspond to National (blue), Labour (red), Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

Party vote support for the six minor NZ political parties

Party vote support for the six minor NZ political parties as determined by moving averages of political polls. Colours correspond to Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

As always, please check the Graphs page for further simulation results.

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It’s been about a month since the last update, and in that time we’ve had six new polls: two polls from Roy Morgan Research (1, 2), and one each from TV3-Reid Research, One News-Colmar Brunton, Herald-DigiPoll and Fairfax Media.

The updated polling averages now have National on 54.0% +/- 1.2%, Labour on 28.9% +/- 1.1%, the Greens on 8.4% +/- 0.6%, NZF on 2.8% +/- 0.5%, ACT on 1.9% +/- 0.4% and the Maori Party on 1.9% +/- 0.3%.

Relative to an earlier polling update from 21 August, about a month ago, Labour is down about 1% +/- 1%. The averages have National and the Greens each up about 0.5% +/- 1% relative to a month or so ago; not statistically significant movements on their own, but consistent with the pattern of them both picking up support from Labour’s downward trend over the last half-year or so.

As usual, the two graphs below summarise the polling averages for the party vote after the new poll. The horizontal axes represent the date, starting 60 days before the 2008 NZ General Election, and finishing on the present day. The solid lines with grey error bands show the moving averages of the party vote for each party, and circles show individual polls with the vertical lines representing the raw statistical errors.

Party vote support for the eight major and minor NZ political parties

Party vote support for the eight major and minor NZ political parties as determined by moving averages of political polls. Colours correspond to National (blue), Labour (red), Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

Party vote support for the six minor NZ political parties

Party vote support for the six minor NZ political parties as determined by moving averages of political polls. Colours correspond to Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

As always, please check the Graphs page for further simulation results.

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Today’s poll is from Fairfax Media. It is the first Fairfax Media poll since early November 2008, just before the previous general election, and looking at their polling behaviour from 2008 we can probably expect about one Fairfax Media poll per month until November.

The updated polling averages now have National on 52.5% +/- 1.1%, Labour on 30.6% +/- 1.0%, the Greens on 7.7% +/- 0.5%, New Zealand First on 2.5% +/- 0.5% and ACT on 2.4% +/- 0.4%. Changes since the last update are not statistically significant, although the recent trend that has Labour down slightly by 2% +/- 1% from a couple of months ago is still visible.

More interestingly, changes since the previous Fairfax poll in 2008 aren’t really statistically significant either. I still can’t see any big changes in polling for most parties since early 2009.

As usual, the two graphs below summarise the polling averages for the party vote after the new poll. The horizontal axes represent the date, starting 60 days before the 2008 NZ General Election, and finishing on the present day. The solid lines with grey error bands show the moving averages of the party vote for each party, and circles show individual polls with the vertical lines representing the total errors.

Party vote support for the eight major and minor NZ political parties

Party vote support for the eight major and minor NZ political parties as determined by moving averages of political polls. Colours correspond to National (blue), Labour (red), Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

Party vote support for the six minor NZ political parties

Party vote support for the six minor NZ political parties as determined by moving averages of political polls. Colours correspond to Green Party (green), New Zealand First (black), Maori Party (pink), ACT (yellow), United Future (purple), and Progressive (light blue) respectively.

As always, please check the Graphs page for further simulation results.

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