The best of these pollsters over this period has been Monmouth University, which has an Advanced Plus-Minus score of -1.5. That’s not a huge surprise — Monmouth was already one of our highest-rated pollsters. After that, the list is somewhat eclectic, including traditional, live-caller pollsters such as Siena College and Marist College, as well as automated pollsters such as Emerson College and Landmark Communications. Polling institutes run by colleges and universities are somewhat overrepresented among the high performers on the list and have generally become a crucial source of polling as other high-quality pollsters have fallen by the wayside.

The lowest-performing pollsters in this group are the University of New Hampshire’s Survey Center, Google Surveys and SurveyMonkey. UNH uses traditional telephone interviewing, but its polls were simply way off the mark in 2016, overestimating Democrats’ performance by an average of almost 9 percentage points in the polls it conducted of New Hampshire and Maine.