Are exit polls results done in the run up to elections always accurate? No, says new research that points out two major lacunae in the whole exercise.
"Excluding the population who only have a mobile phone from pre-election telephone polls leads to significant biases in assessing voter intention," said Sara Pasadas del Amo, in her PhD thesis written at the NUP/UPNA-Public University of Navarre, Spain.
The work has its starting point in the study provided by the regional Andalusian elections of 2012 in which the predictions of all the pre-election polls were wrong. "These polls predicted a distance of the PP (conservative) ahead of the PSOE (socialist) that ranged between 9 and 15 points and which in the end was reduced to little more than one point," she said.
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As Sara Pasadas pointed out, the reasons behind this lack of accuracy can be grouped into two categories. Firstly, factors relating to the behaviour of the people interviewed and the electors, some of whom state one thing in the polls and do something different on the polling day. And secondly, the factors relating to the technical and methodological design of the polls.
In the case of phone interviews, a significant chunk of the population has no chance of being part of the samples, either because they do not have a phone at home or because their numbers do not appear in the telephone number data that many of these surveys use as the sampling frame.
"To this segment of the population that cannot be located by phone, which in itself is high, one would have to add the population that refuses to participate in polls of this type, and who, if they agree to respond, refrain from the questions about voting intention," the researcher wrote.