As many researchers, I am following the polls for this election. I will personnally focus on aspects of the polls that are not that much covered, that is the possible impact of the methodological features of the polls. In this blog post, I focus on whether there is a difference in the trends portrayed by different modes of administration.
As the first figure shows, the polls conducted since the beginning of August tend to portray an estimated difference of eigth points in support between Joe Biden and Donald Trump. We use a common scale for all the polls, that is, support for each candidate on the sum of the support for the two of them. The portrait traced here is not different from the portrait traced by well-known agregators and news media.
By mode of administration
However, as in 2016, the trends traced by the different modes of administration tend to differ. IVR polls, that use a combination of automated phone calls to landline phones and web polls to cell-only users, estimate the proportional support for Donald Trump two points higher than the other polls. This was also the case in 2016 just before the first debate between Donald Trump and Hillary Clinton as shown in the AAPOR Report on the polls of the 2016 election.
As in 2016, the web polls tend to trace a more stable portrait of the trends. They do not seem to detect movement when other modes do detect some. Finally, the polls that use live interviewers show an increase in support for Donald Trump in August, an increase that is also detected by IVR polls.Whatever the mode used however, trends in support for Donald Trump appear stable since the end of August.
Web polls account for more than 70 percent of the polls. Therefore, the portrait that we get if we do not take mode into account is heavily determined by web polls estimates. In 2016 however, the IVR polls had a slightly better estimate of the support for Donald Trump than the two other modes. We need to take this into account.
Of course, there are slso differences within modes. Web pollsters do not use the same sources of potential respondents. These sources may be more or less probabilistic and some combine different sources. This may have an impact. IVR polls also differ according to the pollster who use them, particularly in terms of the proportion of the sample coming from automated calls to landline phones. Finally, the polls conducted using live interviewers differ in the proportion of cell phones in their samples. In 2016, this proportion varied from 25 to 75 percent. I presented here an average impact of modes but there may be differences within modes, a question that I will try to cover in future posts.
A final note to pollsters and journalists. Mode of administration is an essential methodological information. It should be presented with the main information regarding the dates of fieldwork and the sample size. Sometimes, it is hidden somewhere in the pollster report. It shouldn't be.