The 2016 Democratic primary in Michigan was a wakeup call. With every recent survey of the state showing Hillary Clinton ahead by double digits, many deemed a Bernie Sanders victory out of the question — until results came in. The Vermont senator’s 23 percentage point overperformance is now considered one of the biggest polling errors in primary history.
There are many causes behind the record-breaking blunder, but the night’s shortcomings can be attributed partially to a drought of contemporary data and the ensuing uncertainty. Only three surveys of Michigan were released in the week preceding the 2016 primary; four years later, pollsters learned their lesson and conducted twice as many polls beforehand, taking some of the suspense out of Joe Biden’s win earlier in March.
It should come as no surprise that having more polls improves predictions. The relationship is key to choosing where surveys are fielded, because even if it would be nice to have 50 polls of all 50 states, pollsters are understandably interested in some places over others. And while such decision-making is consequential for projecting the outcomes of presidential primaries, it also might have an outsized impact on forecasting the general election.
Based on the final Inside Elections race ratings of the 2016 Electoral College, a state’s “battleground” status largely determined how often it was polled. The five states with the most surveys in the run-up to that November — Florida, North Carolina,…