Synthetic control methods have been widely used for evaluating policy effects in comparative case studies. However, most existing synthetic control methods implicitly rule out interference effects on the untreated units, and their validity may be jeopardized in the presence of interference. In this paper, we propose a novel synthetic control method, which admits interference but does not require modeling the interference structure. Identification of the effects is achieved under the assumption that the number of interfered units is no larger than half of the total number of units minus the dimension of confounding factors. We propose consistent and asymptotically normal estimation and establish statistical inference for the direct and interference effects averaged over post-intervention periods. We evaluate the performance of our method and compare it to competing methods via numerical experiments. A real data analysis, evaluating the effects of the announcement of relocating the US embassy to Jerusalem on the number of Middle Eastern conflicts, provides empirical evidence that the announcement not only increases the number of conflicts in Israel-Palestine but also has interference effects on several other Middle Eastern countries.

翻译：合成控制方法已被广泛用于比较案例研究中的政策效果评估。然而，现有的大多数合成控制方法都隐含地排除了对未处理单元的干扰效应，在存在干扰的情况下，其有效性可能受到影响。本文提出了一种新型的合成控制方法，该方法允许干扰存在，且无需对干扰结构进行建模。在受干扰单元数量不超过总单元数减去混杂因素维度的一半这一假设下，我们实现了对效应的识别。我们提出了一致且渐近正态的估计方法，并建立了针对干预后时期平均的直接效应与干扰效应的统计推断。我们通过数值实验评估了所提方法的性能，并将其与现有竞争方法进行了比较。一项关于美国驻以色列使馆迁至耶路撒冷的公告对中东冲突数量影响的真实数据分析提供了实证证据，表明该公告不仅增加了以色列-巴勒斯坦地区的冲突数量，还对其他几个中东国家产生了干扰效应。