Refer to equations (15.19) and (15.20). Assume that it = x, so that the population variation in

Question:

Refer to equations (15.19) and (15.20). Assume that σit = σx, so that the population variation in the error term is the same as it is in x. Suppose that the instrumental variable, z, is slightly correlated with u: Corr(z, u) = .1. Suppose also that z and x have a somewhat stronger correlation: Corr(z,x) = .2.
(i) What is the asymptotic bias in the IV estimator?
(ii) How much correlation would have to exist between x and u before OLS has more asymptotic bias than 2SLS?