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arXiv 2406.02835econ.EM
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工具变量识别何时不限制结果?

When does IV identification not restrict outcomes?

Leonard Goff

AI总结:

本文给出工具变量模型中处理效应不限制结果被点识别的充要条件,推广LATE单调性,并应用于交叉随机处理的互补性识别。

摘要:

工具变量(IV)模型中的许多识别结果成立时,并不需要对潜在结果的分布施加任何限制,也不需要对结果与选择行为之间的相关性施加限制。这使得工具变量模型能够允许处理效应存在任意异质性,以及结果中存在基于收益的选择可能性。当工具取有限个数值时,我提供了处理效应以不限制结果的方式被点识别的一个必要且充分条件。该条件推广了著名的LATE单调性假设,并统一了多种其他已知的工具变量识别结果。该结果还提供了一种暴力搜索方法,以揭示所有允许在不限制结果的情况下点识别处理效应的选择模型,然后在每个此类选择模型中枚举所有被识别的参数。这一搜索发现了新的可产生识别的选择模型,为其他模型提供了不可能性结果,并为放宽现有文献中使用的选择假设提供了机会。一项应用考虑了两种交叉随机处理之间互补性的识别,获得了关于选择的一个必要且充分条件,使得在编译器(compliers)中局部平均互补性能够以不限制结果的方式被识别。我利用这一结果重新审视了两个实证场景,其中一个场景的数据与这一选择限制不相容,而另一个场景的数据与该限制相容。

英文摘要:

Many identification results in instrumental variables (IV) models hold without requiring any restrictions on the distribution of potential outcomes, or how those outcomes are correlated with selection behavior. This enables IV models to allow for arbitrary heterogeneity in treatment effects and the possibility of selection on gains in the outcome. I provide a necessary and sufficient condition for treatment effects to be point identified in a manner that does not restrict outcomes, when the instruments take a finite number of values. The condition generalizes the well-known LATE monotonicity assumption, and unifies a wide variety of other known IV identification results. The result also yields a brute-force approach to reveal all selection models that allow for point identification of treatment effects without restricting outcomes, and then enumerate all of the identified parameters within each such selection model. The search uncovers new selection models that yield identification, provides impossibility results for others, and offers opportunities to relax assumptions on selection used in existing literature. An application considers the identification of complementarities between two cross-randomized treatments, obtaining a necessary and sufficient condition on selection for local average complementarities among compliers to be identified in a manner that does not restrict outcomes. I use this result to revisit two empirical settings, one in which the data are incompatible with this restriction on selection, and another in which the data are compatible with the restriction.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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