October 25, 2011

All about election polls

November 26 is Election Day, and from now on, you’ll be getting election polls from all directions. So which ones can you trust?  The easy answer is: none of them.  However, some polls are worth more than others.

Assess their worth by asking these questions:

  • Who is commissioning the poll? Is this done by an objective organisation or is it done by those who have a vested interest? Have they been clear about any conflict of interest?
  • How have they collected the opinions of a representative sample of eligible voters? One of the cardinal sins of polls is to get people to select themselves (self-selection bias) to volunteer their opinions, like those ‘polls’ you see on newspaper websites. Here, you have no guarantee that the sample is representative of voters. “None of my mates down at the RSA vote that way, so all the polls are wrong” is a classic example of how self-selection  manifests itself.
  • How did they collect their sample? Any worthy pollster will have attempted to contact a random sample of voters via some mechanism that ensures that they have no idea who, beforehand, they will be able to contact.  One of the easiest ways is via computer-aided interviewing (CATI) of random household telephone numbers (landlines), typically sampled in proportion to geographical regions with a rural/urban split (usually called a stratified random sample). A random eligible voter needs to be selected from that household – and it won’t necessarily be the person who most often answers the phone! A random eligible voter is usually found by asking which of the household’s eligible voters had the most recent birthday and talking to that person.  But the fact that not all households have landlines is an increasing concern with CATI interviewing.  However, in the absence of any substantiated better technique, CATI interviewing remains the industry standard.
  • What about people who refuse to cooperate? This is called non-response. Any pollster should try to reduce this as much as possible by re-contacting households that did not answer the phone first time around, or, if the first call found the person with the most recent birthday wasn’t home, try to get hold of them.  If the voter still refuses, they become a ‘non-respondent’ and attempts should be made to re-weight the data so that this non-response effect is diminished. The catch is that the data is adjusted on the assumption that the respondents selected represented the opinion of a non-respondent on whom, by definition, we have no information. This is a big assumption that rarely gets verified. Any worthy polling company will mention non-response s and discuss how they attempt to adjust for them. Don’t trust any outfits that are not willing to discuss this!
  • Has the polling company asked reasonable, unambiguous questions? If the voters are confused by the question, their answers will be too. The pollsters need to state what questions have been asked and why. Any fool can ask questions – asking the right question is one of the most important skills in polling. Pollsters should openly supply detail on what they ask and how they ask it.
  • How can a sample of, say, 1000 randomly-selected voters represent the opinions of 3 million potential voters? This is one of the truly remarkable aspects of random sampling. The thing to realise is that whilst this a very small sub-sample of voters, provided they have been randomly selected, the precision of this estimate is determined by the amount of information you have collected, not the proportion of the total population (provided this sampling fraction is quite small e.g. 1000 out of 3 million).
  • What is the margin of error (MOE)?  It’s a measure of precision. It measures the price paid for not taking a complete census of the data, which happens once every three years on Election Day, which we call in statistical terms a population result. The MOE is based on behaviour of all similar possible poll results we could have selected (for a given level of confidence which is usually taken to be 95%). Once we know what that behaviour is (via probability theory and suitable approximations) we can then use the data that has been collected to make inference about the population that interests us. We know that 95% of all possible poll results plus or minus their MOE include the true unknown population value. Hence, we say we are 95% confident that a poll result contains the population value.
  • When we see quoted a MOE of 3.1% (from random sample of n=1000 eligible voters), how has it been calculated? It is, in fact, the maximum margin of error that could have been obtained for any political party. It is only really valid for parties that are close to the 50% mark (National and Labour are okay here, but it is irrelevant for, say, NZ First, whose support is closer to 5%). So if National is quoted a having a party vote of 56%, we are 95% confident that the true population value for National support is anywhere between 56% plus or minus 3.1% or about 53% to 59% – in this case, indicating a majority.
  • Saying that a party is below the margin of error is saying it has few people supporting it, and not much else. Its MOE will be much lower than the maximum MOE. For back-of-the-envelope calculations, the maximum MOE for a party is approximately =1/(square root of n), e.g. If n=1000 random voters  are sampled then  MOE  1/(square root of 1000) =1/31.62 =3.1%.
  • Comparing parties become somewhat more complicated.  If National are up, then no doubt Labour will be down. So to see if National has a lead on Labour, we have to adjust for this negative correlation. A rough rule of thumb for comparing parties sitting around 50% is to see if they differ by more than 2xMOE. So if Labour has 43% of the party vote and National 53% (with MOE = 3.1% from n=1000) we can see that this 10% lead is greater than 2×3.1=6.2% – indicating that we have evidence to believe that this lead of National is ‘real’, or statistically significant.
  • Note that any poll result only represents the opinion of those sampled at the place and time. As a week is a long time in politics, and most polls become obsolete very quickly. Note also that poll results now can affect poll results tomorrow, so these results are fluid, not fixed.

If you’re reading, watching or listening to poll results, be aware of their limitations. But note that although polls are fraught with difficulties, they remain useful. Any pollster who is open about the limitations of his or her methods is to be trusted over those who peddle certainty based on uncertain or biased information.


Andrew Balemi is a Professional Teaching Fellow in the Department of Statistics at The University of Auckland. He is a former Head of Marketing Science at market research company Colmar Brunton. See all posts by Andrew Balemi »


  • avatar

    When pollsters ask questions such as “preferred PM” or policy preferences, do they do this after the key “who will you vote for at the election” question?

    If they did so before, that would skew the poll – these questions will not be asked on the day.

    Also, I’d think that a model that takes an unrepresentative sample and “adjusts” it will work and produce correlations with polling day until there’s some sort of step change in the way people vote – at that stage the corrections will be wrong and it’ll fall over.

    6 years ago

  • avatar

    I liked that you included “How can a sample of, say, 1000 randomly-selected voters represent the opinions of 3 million potential voters?” as that comes up *so* many times.

    Another one which comes up a lot along similar lines is an individual saying “The results are wrong because everyone I know says…” (i.e. a person assuming the people they know are a representative sample).

    6 years ago

  • avatar

    Oh wait, you did mention that one in the RSA comment :)

    6 years ago

  • avatar
    Chuck Bird

    Good article Andrew. Do know the questions asked in the resent Epson poll? It would appear the result that were published were the ones that got the desired result.

    6 years ago

  • avatar

    Rich: It is standard for pollsters to ask the party vote question first, for the reason you outline.

    One new company that does online panels only, asks it at the very end of an often extensive survey on political issues. They are the exception, not the rule.

    6 years ago

  • avatar
    Caroline Glass

    are the polls weighted in some way to take account of the size of a household? it seems to me that, now that I live alone, I have 6 times the chance of being polled as I did when I lived in a student flat with 5 flatmates.

    3 years ago

    • avatar
      Thomas Lumley

      I think so. Colmar Brunton poll reports say “The data have been weighted to align with Statistics New Zealand population counts for age, gender, household size and ethnic identification.”

      This gets trickier when you call mobile phones as well as landlines, as in some large health surveys, but it can still be done.

      3 years ago