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We hear a lot these days about “evidence-based policy.” As a university professor I’m certainly not going to argue against evidence. The more we have, the better — depending on what “more” costs, that is, for a first principle of economics is that you can have too much of a good thing.

But “evidence-based policy” has a scientific ring to it that often isn’t warranted. People hear “evidence-based” and they think, “Your honour, we have video of the defendant shooting the victim. His fingerprints and DNA are on the gun, which is registered in his name, and we found gunpowder on his fingers. His bank records show he owed the victim $100,000.” It’s a slam dunk.

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The evidence we’re dealing with in most policy contexts isn’t like that (not to mention that court evidence often isn’t like that, either!) Usually it’s correlation analysis, using data that’s never exactly the data you want collected exactly the way you want it collected. And most of the situations you’re examining are real life, not controlled experiments. Lots of other things are going on that may effect your correlations but that you can’t control for.