All norm enforcement regimes, formal and informal, face an inevitable tradeoff between false negatives and false positives.
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Unfortunately in this case you don't usually have the true underlying data to compare against. But there are people who have been executed later proved innocent, and presumably people acquitted with later evidence implying guilt.
I don’t suggest reading this whole thing, but I’m happy it exists http://www2.law.ucla.edu/volokh/guilty.htm …