Abstract

We use the exchange between Kearney/Levine and Jaeger/Joyce/Kaestner on 16 and Pregnant to reexamine the use of DiD as a response to the failure of nature to properly design an experiment for us. We argue that (1) any DiD paper should address why the original levels of the experimental and control groups differed, and why this would not impact trends, (2) the parallel trends argument requires a justification of the chosen functional form and that the use of the interaction coefficients in probit and logit may be justified in some cases, and (3) parallel trends in the period prior to treatment is suggestive of counterfactual parallel trends, but parallel pre-trends is neither necessary nor sufficient for the parallel counterfactual trends condition to hold. Importantly, the purely statistical approach uses pretesting and thus, generates the wrong standard errors. Moreover, we underline the dangers of implicitly or explicitly accepting the null hypothesis when failing to reject the absence of a differential pre-trend.

摘要

我们使用16岁以下儿童和孕妇的Kearney / Levine和Jaeger / Joyce / Kaestner之间的交流重新检查DiD的使用，以应对自然界的失败，以便为我们正确设计实验。我们认为（1）任何DiD论文都应说明实验组和对照组的原始水平为何不同，以及为什么这不会影响趋势；（2）平行趋势论证要求选择的功能形式合理，并且使用概率和对数中的相互作用系数在某些情况下可能是合理的，并且（3）治疗前一段时间的平行趋势暗示了反事实的平行趋势，但是对于平行的反事实趋势条件，平行的预趋势既没有必要，也不充分举行。重要的是，纯统计方法使用预测试，因此会产生错误的标准误差。此外，