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.

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