538: Calm Down About Those Virginia Polls, Folks [View all]
A Monmouth University poll released Tuesday generated a ton of headlines. It found Republican Ed Gillespie leading the Virginia gubernatorial race by 1 percentage point, 48 percent to 47 percent. Democrat Ralph Northam has led in most surveys of the race, and if Republicans win Virginia with President Trump so unpopular, you can expect a full-blown freakout among Democrats. Adding to the confusion: Quinnipiac University released a poll on Wednesday showing Northam up 14 percentage points, 53 percent to 39 percent.
So what the heck is going on in the Virginia governor’s race? Nothing. The split between the Monmouth and Quinnipiac results is big, but it’s not unnatural. In fact, it’s a sign that pollsters are doing their job.
Polling averages work best when pollsters are working independently. You have different pollsters using different methods and making different estimates of the electorate, and you get a more accurate picture of the race by averaging their results together than by looking at any individual poll. It’s kind of like the old “wisdom of the crowd” principle.
That doesn’t work if pollsters “herd” — which my colleague Nate Silver defined as “the tendency of polling firms to produce results that closely match one another.” When pollsters release results that are closer to each other than is statistically plausible, it may make individual polls more accurate, but it makes the average less so. That is, there should be a big spread among polls of the same race. Unfortunately, herding happens, particularly as Election Day approaches.
https://fivethirtyeight.com/features/calm-down-about-those-virginia-polls-folks/?ex_cid=story-facebook
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