Trump fares better in polls conducted by robots; Sanders polls better when humans conduct the interview. Sanders also shows greater strength in polls of “likely” voters.
Commentators often treat Donald Trump and Bernie Sanders as representing two “insurgencies” within the Republican and Democratic Parties. While there are certainly some surface similarities between the two candidacies, national polling data for the two candidates show substantial differences as well. I begin with two charts comparing the trends in their national polling support using data from Huffington Post Pollster since each candidate announced he was running for President of the United States.
While both men’s support has continued to grow over the course of the campaign, the trajectories of their support are radically different. Sanders’ polling figures have increased consistently over the course of the campaign, but the rate at which his support has risen has slowed as the campaign progressed. Trump’s support seems to have gone through three phases — rapid growth at the outset, a plateau over the fall, and a second surge beginning around Thanksgiving that slowed at the turn of the year. Extrapolating out to February 1st when the Iowa Caucuses take place, Trump would be approaching just under forty-five percent in national polls with Sanders reaching thirty-five percent.
I have examined two types of polling effects: the method of interviewing and the type of sample drawn. Trump does over four percent worse when interviews are conducted by a live human being. Sanders does worse by an essentially identical margin in polls that use automated telephone methods. The result for Trump may reflect an unwillingness on the part of his supporters to admit to preferring the mogul when talking with an actual human interviewer.
Sanders’ poorer showing in polls that rely on automated polling may have to do with their exclusion of cell phones which cannot by law be called by robots. Usually this problem is adjusted for after the poll has been conducted by weighting the data to conform to expected demographic breakdowns. However Sanders large lead among younger voters who are much less likely to have a landline phone may be suppressing his support in automated polling. In a recent FoxNews poll, for instance, Sanders holds a 61-31 lead over Hillary Clinton among respondents under 45 years of age; older voters strongly prefer Clinton 71-21. That same demographic explanation does not work for Trump, however, since he drew an identical 35 percent among voters in that same poll from both age groups. The “social desirability” explanation probably has greater force when accounting for his poorer showing in polls conducted by human interviewers.
Turning to the differences in sampling frames, we find that polls that screen for “likely” voters show surprisingly greater levels of support for Bernie Sanders than do polls that include all registered voters or all adults. Trump’s support shows no relationship with the sample of voters drawn. Both candidates, as “insurgents,” are thought to suffer from the problem of recruiting new, inexperienced voters who might not actually show up at the polls for primaries and caucuses. That seems not to be an issue for either man, and in Sanders’ case it appears that the enthusiasm we have seen among his supporters may well gain him a couple of percentage points when actual voting takes place.