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背景：工资与就业密度之间具有内生性。为了解决这个问题，分两步法进行。
原文（红颜色的为下标）：
This wage can then be regressed on a number of (mean) characteristics of the workers and the local sector. More specially, we can estimate the following first-step regression:
(12) ln wast= Wast + rs + b*Xast + εast
In this equation, r is a sector dummy, and Xast is a set of characteristics for sector s in area a and year t and the workers employed therein. To capture sector effects, we use in Xast the (log) share of local employment in sector s and the (log) number of local establishments in this sector. Also in Xast, the mean individual characteristics are the age, its square, and the shares of employment in each of six skill groups. In equation (12), the coefficient of interest is Wast, a fixed effect for each employment area and year. When at estimating equation (12), all local sector and mean individual characteristics are centered, and the observations are weighted by the number of workers in each cell to avoid heteroscedasticity.
The coefficients Wast can, in a second step, be regressed on local employment density and other local characteristics.
我的问题是：
1.第一步，公式（12）中的Wat怎么估计，怎么得到它的具体数值呢？
2.为了处理异方差，文中说乘以工人的数量，是不是（12）式中每个变量都乘以地区a产业s时间t上的工人数量？
3.第二步，用第一步得到的Wat对就业密度做回归。请问：这种两步法估计有什么好处？处理成一步可以吗？
您非常忙碌，给您带来的不便请谅解，感谢您。祝节日快乐！
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