Exit Polls and Base Rates

I’ve seen this problem crop up a couple of times, but I think this is the most egregious so far. After every contest, the media has to come up with some sort of “narrative” to explain what happened the night before. CNN’s coverage has been typically good, and they’ve done a “Five Things We Learned Last Night” piece after nearly every primary so far. Today’s piece, about Wisconsin, had this to say:

Santorum’s coalition fell apart

Wisconsin Republicans didn’t just side with Mitt Romney on Tuesday — they rejected Rick Santorum…Evangelicals, tea party supporters, those supporting “traditional values” and people calling themselves “very conservative” went Romney’s way, exit polls showed.

The problem that I’ve seen again and again with these exit polls is that they don’t take base rates into account. Mitt Romney got 42% of ALL voters. According to the detailed exit polls on CNN, he got 43% of voters calling themselves “very conservative” – the same number that he got, on average, from any other group. And Santorum ALSO got 43% of “very conservative” voters – 5 points OVER his total, which means that “very conservative” voters were much more important to his base than to Romneys.

It’s just wrong that evangelicals went Romney’s way – Santorum got those votes 43-38. To me, the importance of exit polls is showing where a candidate is weak or strong relative to their overall support in the population.

Romney had a great night last night that took him a couple steps closer to locking up the nomination – but half-way into the contest, he’s still winning key states like Wisconsin by only 4 percentage points. That’s not exactly a game-changing night.

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About thinklikeafox

I'm a Naval Officer living in Southern California. I hope to be attending law school in the next year or two, and I started writing this blog out of a desire to improve my writing and critical thinking skills after a couple years outside of academia.
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