The stable unit treatment value (SUTVA) is a crucial assumption in the Difference-in-Differences (DiD) research design. It rules out hidden versions of treatment and any sort of interference and spillover effects across units. Even if this is a strong assumption, it has not received much attention from DiD practitioners and, in many cases, it is not even explicitly stated as an assumption, especially the no-interference assumption. In this technical note, we investigate what the DiD estimand identifies in the presence of unknown interference. We show that the DiD estimand identifies a contrast of causal effects, but it is not informative on any of these causal effects separately, without invoking further assumptions. Then, we explore different sets of assumptions under which the DiD estimand becomes informative about specific causal effects. We illustrate these results by revisiting the seminal paper on minimum wages and employment by Card and Krueger (1994).

翻译：稳定单元处理值（SUTVA）是双重差分（DiD）研究设计中的一个关键假设。它排除了隐藏的处理版本以及单元间的任何干扰和溢出效应。尽管这是一个强假设，但并未受到DiD实践者的足够重视，且在多数情况下，甚至未被明确表述为一项假设——尤其是无干扰假设。在本技术报告中，我们探究了在存在未知干扰的情况下，DiD估计量所识别的对象。我们证明，DiD估计量识别的是因果效应的对比，但若不引入进一步假设，它无法单独提供关于其中任一因果效应的信息。随后，我们探讨了在不同假设集合下，DiD估计量如何能够提供关于特定因果效应的信息。我们通过重审Card和Krueger（1994）关于最低工资与就业的开创性论文来阐释这些结论。