论文标题
自然实验
Natural Experiments
论文作者
论文摘要
自然实验一词不一致。在一种解释中，它是指一个实验，该实验是由研究人员以外的其他人随机分配的。在另一种解释中，它指的是一项研究，其中没有受控的随机分配，但是某种外部因素以松散类似于随机实验的方式分配了治疗 - 通常被描述为“仿佛随机”分配。在另一种解释中，它指的是将治疗与对照组进行比较的任何非随机研究，而对治疗方式的分配方式没有任何具体要求。我介绍了一个替代定义，该定义旨在阐明自然实验的整体特征，同时将它们与随机对照实验区分开。我将自然实验定义为一项研究研究，其中研究人员既不设计也不实施治疗分配机制（i），研究人员未知，并且（iii）由于外部因素而依赖于概率。该定义的主要信息是，随机对照实验和自然实验之间的差异不是程度的问题，而是本质上的问题，因此将自然实验概念化为类似于随机实验的研究设计，既不是严格的，也不是经验分析的有用指南。使用我的替代定义，我讨论了自然实验与传统观察性研究的不同，并为希望使用自然实验来研究因果效应的研究人员提供实用建议。
The term natural experiment is used inconsistently. In one interpretation, it refers to an experiment where a treatment is randomly assigned by someone other than the researcher. In another interpretation, it refers to a study in which there is no controlled random assignment, but treatment is assigned by some external factor in a way that loosely resembles a randomized experiment---often described as an "as if random" assignment. In yet another interpretation, it refers to any non-randomized study that compares a treatment to a control group, without any specific requirements on how the treatment is assigned. I introduce an alternative definition that seeks to clarify the integral features of natural experiments and at the same time distinguish them from randomized controlled experiments. I define a natural experiment as a research study where the treatment assignment mechanism (i) is neither designed nor implemented by the researcher, (ii) is unknown to the researcher, and (iii) is probabilistic by virtue of depending on an external factor. The main message of this definition is that the difference between a randomized controlled experiment and a natural experiment is not a matter of degree, but of essence, and thus conceptualizing a natural experiment as a research design akin to a randomized experiment is neither rigorous nor a useful guide to empirical analysis. Using my alternative definition, I discuss how a natural experiment differs from a traditional observational study, and offer practical recommendations for researchers who wish to use natural experiments to study causal effects.
