Source: From exit poll breakdowns based on polling by Edison Research.

But that doesn't tell us why people voted the way they did. On average, for example, black women also have lower incomes, are younger, and have completed less education than whites—and they have worse healthcare.

So did black women vote for Obama because they are proud of him as a black man and identify with his personal experience, or because his policies are more beneficial for them as workers, concerned family members, or medical patients?

And the closer a group gets to the 50/50 middle of the odds breakdown, the harder it is to guess what's going on from the demographics. When the plane I was on Tuesday night landed, I turned on my iPhone to see who had won Ohio. Waiting for the page to load, I looked around and saw a guy a few rows back, smiling and giving me a thumbs up. He must have noticed I'm a white man, almost two-thirds of whom went for Romney. But did he think I was Jewish, or gay? Maybe it was my rumpled casual-business wear, shaggy hair, and dead-giveaway academic backpack. Or my facial expression.

In real life, even with people we don't know personally, the cues we get from interactions and behavior explain more than simple demographics, although they all go into the quick mix of judgments we are forced to form on short notice throughout the day. In statistical terms, the basic demographic variables leave a lot of variance unexplained.

These demographic categories are useful for prediction at the group level, as punditry has proved. Show me a room full of randomly-chosen Mormons and I can guess that 78 percent of them voted for Romney. But give me 30 seconds with one of them personally and I might figure out whether she is the one who didn't.

This paradox is why the "big data" people were so central to the campaigns. If you can get someone to give you a few key bits of information—even just a zip code—and then track them as they wander around the web after leaving your site, your models can be much more powerful than the demographics alone. It's the equivalent of sizing up someone by their shoes, haircut, and the flight they're on, rather than just sex and race.

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