We illustrate for two U.S. real estate markets how spillover effects may contaminate a causal impact analysis that relies on a research design with a control unit. Our two markets (Chattanooga, TN and Spartanburg, SC) are linked via dominant local employers (VW and BMW) that compete with each other in the automotive sector. The spillovers originate with an unanticipated negative news event for VW (2015 emissions scandal), which made the company’s very survival uncertain for a limited time period. The troubling news for VW translated into favorable news for BMW’s competitive position. In VW’s location, the demand for homes and prices declined temporarily; in BMW’s location the reverse occurred. A standard impact analysis of the VW scandal on the local housing market in Chattanooga using BMW’s Spartanburg location as a control would have overestimated its impact as a result of the spillover effects.

我们针对两个美国房地产市场说明了溢出效应如何影响依赖于带有控制单元的研究设计的因果影响分析。我们的两个市场（田纳西州查塔努加和南卡罗来纳州斯巴达堡）通过在汽车行业相互竞争的主要当地雇主（大众和宝马）联系在一起。溢出效应源于大众意外的负面新闻事件（2015 年排放丑闻），这使得该公司的生存在有限的时间内充满不确定性。对于大众来说，令人不安的消息转化为对宝马竞争地位有利的消息。在大众所在的地区，房屋需求和价格暂时下降；在宝马的位置，情况正好相反。