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：稳定单位处理值（SUTVA）是双重差分法（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处理单元之间的任何形式的干扰和溢出效应。尽管这是一个强假设，但并未受到DiD实践者的充分关注，且在多数情况下，该假设（尤其是无干扰假设）甚至未被明确陈述。在本技术报告中，我们研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，但若不引入进一步的假设，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估计量如何能够为特定因果效应提供信息。我们通过重新审视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。