The results above come from the 145 national Republican primary polls as archived by Huffington Post Pollster whose fieldwork was completed after June 30, 2015, and on or before January 6, 2016. I started with July polling since the current frontrunner, Donald Trump, only announced his candidacy on June 16th. For Bernie Sanders I used the 155 national polls of Democrats starting after April 30th, the day Sanders made his announcement.
The models I am using are fundamentally similar to those I presented for the 2012 Presidential election polls and include these three factors:
- a time trend variable measured as the number of days since June 30, 2015;
- a set of “dummy” variables corresponding to the universe of people included in the sample — all adults, registered voters, and “likely” voters as determined by the polling organization using screening questions; and,
- a set of dummy variables representing the method of polling used — “live” interviews conducted over the phone, automated interviews conducted over the phone, and Internet polling.
Trump’s support is best fit by a “fourth-order polynomial” with a quick uptick in the summer, a plateau in the fall, and a new surge starting around Thanksgiving that levelled off at the turn of the year. Support for Sanders follows a “quadratic” time trend. His support has grown continuously over the campaign but at an ever slower rate.
Of more interest to students of polling are the effects by interviewing method and sampled universe. Trump does over four percent worse in polls where interviews are conducted by a live human being. Sanders does worse 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 controversial mogul when talking with an actual human interviewer.
Sanders does not suffer from this problem, but polls relying on automated telephone methods show results about four percent lower than those conducted by human interviewers or over the Internet (the excluded category represented by the constant). Since we know that Sanders draws more support from younger citizens, the result for automated polling may represent their greater reliance on cell phones which cannot by law be called by robots. This result contradicts other studies by organizations like Pew that find only limited differences between polls of cell phone users and those of landline users. Nevertheless when it comes to support for Bernie Sanders, polls that rely exclusively on landlines appear to underestimate his levels of support.
Turning to the differences in sampling frames, we find that polls that screen for “likely” voters show 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 universe of voters being surveyed. 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’s 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.
Finally it is clear that Trump’s polling support shows much more variability around his trend line than does Sanders’s. The trend and polling methods variables account for about 59 percent of the variation in Trump’s figures, but fully 72 percent of the variation for Sanders.
