假设1：两期；
假设2：indpvar严格外生。

假设1满足，FE 等价于FD; 假设2 可已不满足。

assume
yit=xit*b+eit, i=1,,,,n; t=1,2

yiba=(yi1+yi2)/2
xiba=(xi1+xi2)/2

FE (using within estimator):
yit-yiba=(xit-xiba)*b+miu
that is for t=1
(yi1-yi2)/2=(xi1-xi2)*b/2+miu1    (1)
for t=2
(yi2-yi1)/2=(xi2-xi1)*b/2+miu2    (2)

(1) and (2) are equivlent and all can be see as : yi2-yi1=(xi2-xi1)*b+miu3

FD:
yi2-yi1=(xi2-xi1)*b+miu4

如果假设2 不满足：两个残差项都是两期差值，对x都有同样的影响；这样两个都得考虑IV问题