占位，更新。。。

研究目标

Research has attempted to measure thebenefits from cities. This article discusses the econometric challengesassociated with this endeavor and how successful it has been.

3 the sources of estimate bias

从协方差推导出存在的有偏估计问题，思路很清晰。

3.1 Endogenous quantity oflabor

3.2 Endogenous quality of labor

3.3 shocks

4 怎么解决

4.1 fixed effects

这里是我见过的评判FE最为详尽和清楚的表达。问题的根源在于：差分后比差分前，使得误差项与自变量更相关了。

4.2 Instrument variables：历史、地理和地质变量

4.3 modeling location choices explicitly

4.4 Quasi-experimental evidence

4.5 firm selection and firm sorting

只列提纲，我觉得已经足够清晰了。具体看文章。

没看懂，需要讨论的部分

4.3 modeling location choices explicitly
Ideally, the location choice of workers would need to be modeled and become part of the estimation strategy to deal property with the endogeneity of individual location choices in the wage equation.
This is an extremely hard thing to do on two counts.
第一个困难The first difficulty is that a choice of location is discrete in nature and this choice is between a large number of locations. Even in a static framework, this raises serious difficulties that can only be overcome through a demanding strategy as demonstrated by Dahl (2002).


第二个困难The second difficulty is perhaps even greater. Proper identification of both a location choice model and a wage equation requires finding a variable that would explain the location without having an effect on the wage.

三条途径
Three short-cuts are possible.
The first is to note that the location choice equation is non-linear whereas the wage equation is. This implies that one can potentially identify the coefficient alpha in such a system. But this would come entirely from the functional form and is unlikely to be convincing.
A more promising alternative is to put more structure behind both the location choice and the determination of wage before structurally estimating such a model like Baum-Snow and Pavan (2010). The quality of the estimation then rests on the quality of the model.
The third possibility is to look for particular situations where mobility is artificially restrained or forcibly organized. Au and Henderson (2006) use mobility restrictions in China to assess net agglomeration effects.

评议
从文章中学到什么
1工资是由供求双方决定的。建立结构方程是十分必要的。具体可见我之前讨论的一篇文章，里面专门讨论了结构方程问题。https://bbs.pinggu.org/thread-2929774-1-1.html

2 文章从基本的协方差出发，讨论了三类内生性问题，非常清晰。

3 讨论了多种解决的途径。其中，对FE的批判，是我见过的写的最清晰的。

4 作者对区位选择的分析，可谓切中内生性问题的要害。