woodridge的书上有介绍

微观计量经济学要义（林少宫）：多元线性回归系数的“其余情况不变”释义 一文中有详细的介绍，可参阅。

路过。。。。。。。。。。

这么回答用你写啊？

An instrumental variable is used to help estimate a causal effect (or to alleviate measurement error). The instrumental variable must affect the independent variable of interest, and only affect the dependent variable through the independent variable of interest. This is called an exclusion restriction.

A proxy variable is a variable you use because you think it is correlated with the variable you are really interested in, but have no (or poor) measurement of.