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What’s in Nate Silver’s magic sauce?

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NEW YORK — The results are in: Nate Silver has won the 2012 presidential election by a landslide. His magic formula for predictions, much maligned in some corners in recent weeks, appears to have hit the mark in every state — a perfect 50 green M&Ms for accuracy. Now my Twitter feed is blowing up with announcements of his coronation as the Emperor of Math and the ruler of the punditocracy. Wait — it was even more than that, they say: a victory for blogging, and also one for rational thought. He proved the haters wrong! He proved science right! Is this guy getting lucky tonight or what?

But all these stats triumphalists have it wrong. The New York Times’ Nate Silver didn’t nail it; the pollsters did. The vaunted Silver "picks" — the ones that scored a perfect record on Election Day — were derived from averaged state-wide data. According to the final tallies from FiveThirtyEight, Obama led by 1.3 points in Virginia, 3.6 in Ohio, 3.6 in Nevada and 1.9 in Colorado. He won all those states, just like he won every other state in which he’d led in averaged, state-wide polls. That doesn’t mean that Silver’s magic model works. It means that polling works, assuming that its methodology is sound, and that it’s done repeatedly.

To be fair, the art of averaging isn’t simple. You can’t just comb through Google News and add up all the numbers that you find; to find a useful sum, Silver judges which polls to put into his analysis and then he weighs them according to his perception of their quality. He may be very good at this, but other stat-head pundits do more or less the same, and their averages match up accordingly. Yes, Silver had Obama in the lead, but so did RealClearPolitics and Talking Points Memo; where he had Romney, so did they. "Our state-by-state forecasts are extremely similar to those issued by our competitors," he wrote in a post two weeks ago, entitled "State Poll Averages Usually Call Election Right." Those numbers nailed it as they often do.

So picking winners state by state was the easy part. Anyone who glossed the numbers would have made the same projections. But Silver’s model promised more than that: He offered assessments of his confidence in each state’s results. The fact that Obama led in Ohio polls made it obvious that he should be the favorite, but what if those Ohio polls were wrong? How much risk was there in trusting state-wide averages? This was Silver’s nifty contribution: He assigned that risk a probability, by looking at some other factors, such as polling trends and local demographics. Take the example of Virginia, where Obama led by 1.3 percentage points. Picking him to win the state was a no-brainer since he was leading in the polls, but Silver used his secret sauce to calculate the chances that those polls were wrong. According to his calculations, the risk was 21 percent, meaning that Obama’s odds to win the state were roughly 4-to-1.

What do the day’s returns tell us about the accuracy of Silver’s model? Nothing much. The fact that Obama won Virginia looks good for averaged polling — indeed, his margin appears to be a couple points, not far off from what was predicted — but we’ll never know about that other part. Did Obama really have a 79 percent chance of winning? To get a sense of that, we’d need to run the election like a lab experiment, doing it 10,000 times to see how often Obama wins. Since that can’t happen, we’re left to scratch our heads.

 

— Slate

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