题目

英语翻译
As we argue in the introduction (and verify below),Pi and Si are negatively correlated:the larger a country is,the farther its typical resident is from other countries.Thus if we do not control for size in (4),Pi will be negatively correlated with the residual,and thus will not be a valid instrument.Intuitively,smaller countries may engage in more trade with other countries simply because they engage in less within-country trade.This portion of the geographic variation in international trade cannot be used to identify trade's impact on income.Similarly,if we fail to control for Ti in (4),Si will be negatively correlated with the residual.Thus,our approach requires us to examine the impacts of both international trade and country size.

提问时间：2020-08-06

答案

这个不难 就像我们在绪论主张的(并于下面证明）,pi和si是负相关的：一国越大,则来自其他国家的常驻民越多,因此倘若我们不控制（4）的规模,pi将与残余值负相关,其自身就不再是一个有效因素了.显然,较小的国家与其他国...