cortes （2011）的论文 low-skilled immigration and the labor supply of highly skilled women
其中介绍工具变量时说：“First, cities that attracted more immigrants in 1970 might be systematically different from other cities, violating the identification assumption. To address this concern, we include specifications that allow cities to experience different decade shocks based on their 1970 value of key variables, such as female educational attain- ment distribution, female labor force participation, and industry composition。”这句话怎么理解呢？