http://fivethirtyeight.com/features/election-update-swing-state-polls-and-national-polls-basically-say-the-same-thing/?ex_cid=538twitterElection Update: Swing State Polls And National Polls Basically Say The Same ThingBy Nate Silver
July 5, 2016
Welcome to our first Election Update, FiveThirtyEight’s regular feature where we’ll, uh, update you about the presidential election. More specifically, we’ll use this column to look at the election through the lens of FiveThirtyEight’s forecasting models, which we launched last week.
If you read FiveThirtyEight in 2008 or 2012, you might remember that we used to update our forecasts once a day, usually in the late afternoon or early evening. Then we’d write a post like this one to accompany it. The timing meant our forecasts were often half a day behind as new polls came in.
So this year, we’ve switched over to running model updates as new data becomes available, sometimes several times per day. I won’t promise you that we’ll interrupt everything to run an update if an Idaho poll drops at 1 a.m., but this method should allow us to stay more up-to-date, especially during regular working hours.
We’ll still run these Election Update columns — a couple of times a week at first and, eventually, almost every weekday. However, their focus will often be more macro than micro — before Labor Day, it usually isn’t worth sweating individual polls. We’ll tend to focus on big-picture themes instead.
Here’s one theme that I expect us to revisit repeatedly, for instance: How are Hillary Clinton and Donald Trump polling in swing states relative to their national numbers?
This is a trickier question than you might think, in part because there’s more than one way to define “national numbers.” One way is to calculate an average of national polls. As of late Tuesday evening, for instance, Clinton led Trump by 5.3 percentage points in our adjusted average of national polls, according to our polls-only forecast. But go to the polls-only homepage, and you’ll find that the forecast has Clinton beating Trump by 6.3 percentage points instead. It’s not a huge discrepancy, but what accounts for the difference?
The reason is that our forecast models use both state and national polls to estimate where the national race stands. In fact, they put most of the emphasis on the state numbers. Historically, it’s been more accurate to take a “bottom-up” approach — first, forecast the vote in each state, then sum the numbers together — than to force everything to match the national numbers. (See the users guide to the forecast for the gory details on how the methodology works.)
So that means Clinton’s swing-state numbers must be really good, right? They’re pretty good — but not great, although she’s gotten some better numbers in the past week. Instead, her biggest comparative strength — and Trump’s biggest comparative weakness — comes from red states rather than swing states.
Here, for instance, are the numbers in red states, which I’ll define as every state that was more Republican-leaning than North Carolina in 2012. (Georgia, by this reckoning, is the bluest red state.) To keep things relatively simple, these figures show FiveThirtyEight’s unadjusted polling average in states where there’s been at least one poll since November, and compare it to President Obama’s result against Mitt Romney in 2012.
Trump is substantially underperforming Romney in red states
Romney won these states by an average of 16 percentage points in 2012, weighted by their turnout. By contrast, Trump leads them by an average of only 7 percentage points, a net swing of 9 points toward Clinton. She’s competitive in a few of these states, such as Georgia, Arizona and — more exotically — Utah and Kansas. If you saw polls from these states only, they’d be suggestive of a double-digit landslide against Trump.
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