The The Iron Law of Evaluation (Rossi, 1987) is that the expected value of any net impact assessment of any large scale social program is zero.

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Why does this happen? The simple answer is that in a largely-rich, largely-free country, with many existing (if confusing) private and public supports for low-income people, it’s just as easy to screw things up than to make things better, no matter how much you spend.

Source: The Iron Law – spottedtoad