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How much does an impact evaluation improve policy decisions?

Thanks to excellent feedback, I’ve extended my generalizability paper to include discussion of how much an impact evaluation improves policy decisions.

Results, in a nutshell: the “typical” impact evaluation (of a program with a small effect size, compared to an outside option that also has a small effect size) might improve policy decisions by only about 0.1-0.3% (of a small amount). If the outside option is much different (say an effect size of 0) and it is one of the earliest impact evaluations on a topic, this can go up to 4.6%.

There are a lot of caveats here, chief among them that an impact evaluation provides a public good and many people can use its results.

Nonetheless, personally, I find this sobering. I don’t think we’re usually in that best case scenario. These aren’t the results I want, but they are the results I get.

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