For whom the belle polls
The TV piece concluded by lambasting a recent One News text poll, saying: “A One News text poll giving the debate to Mr Key 61-39 has been widely discredited, since it cost 75c to vote.”
This text poll should be lambasted if it is used to make inference about the opinions of the population of eligible willing voters. Self-selection is the major problem here: those who can be bothered have selected themselves.
The problem: there is no way of ascertaining that this sample of people is a representative sample of willing voters. Only the interested and motivated, who text message, have answered and, clearly, the pollsters do not have information on the not-so-interested and motivated non-texters.
The industry standard is to randomly select from all eligible willing voters and to adjust for non-response. The initial selection is random and non-response is reduced as much as possible. This is to ensure the sample is as representative of the population as possible. The sample is collected via a sampling frame which, hopefully, is a comprehensive a list of the population you wish to talk to. For CATI polls the sample frame is domestic landline (traditional) phone numbers.
With election polls, as landlineless voters have been in lower socio-economic groups which tend to have lower voter participation, this has not been so much of a problem. The people we wish to talk to are eligible willing voters – so the polls have not been unduly biased by not including these landlineless people.
However, as people move away from landlines to mobile phones, CATI interviewing has been increasingly criticised. Hence alternatives have been developed, such as panel-polls and market prediction polls like IPredict – and the latter will be the subject of a post for another day.
But let’s go back to the Horizon panel poll mentioned above. It claims that it’s to be trusted as it has sampled from a large population of potential panellists who have been recruited and can win prizes for participation. The Horizon poll adjusts for any biases by reweighting the sample so that it’s more like the underlying New Zealand adult population – which is good practice in general.
However, the trouble is this large sampling frame of potential panellists have been self-selected. So who do they represent?
To illustrate, it’s hard to imagine people from more affluent areas feeling the need to get rewards for being on a panel. Also, you enrol via the internet and clearly this is biased towards IT-savvy people. Here the sampling frame is biased, with little or no known way to adjust for any biases bought about from this self-selection problem. They may be weighted to look like the population but they may be fundamentally different in their political outlook.
Panel polls are being increasingly used by market researchers and polling companies. With online panel polls it’s easier to obtain samples, collect information and transfer it, without all the bother involved in traditional polling techniques like CATI.
I believe the industry has been seduced by these features at the expense of representativeness – the bedrock of all inference. Until such time as we can ensure representativeness, I remain sceptical about any claims from panel polls.
I believe the much-maligned telephone (CATI) interviewing, which is by no means perfect, still remains the best of a bad lot.
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 »