Category THE PATH FROM CAUSE TO EFFECT

LATE for Charter School

The KIPP lottery exemplifies an IV chain reaction. The components of such reactions have been named, so masters can discuss them efficiently. We’ve noted that the original
randomizer (in this case, a KIPP offer) is called an instrumental variable or just an instrument for short. As we’ve seen, the link from the instrument to the causal variable of interest (in this case, the effect of lottery offers on KIPP attendance) is called the first – stage, because this is the first link in the chain. The direct effect of the instrument on outcomes, which runs the full length of the chain (in this case, the effect of offers on scores), is called the reduced form. Finally, the causal effect of interest—the second link in the chain—is determined by the ratio of reduced form to first-stage estimates...

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