In this paper I discuss three issues related to bias of OLS estimators in a general multivariate setting. First, I … discuss the bias that arises from omitting relevant variables. I offer a geometric interpretation of such bias and derive … sufficient conditions in terms of sign restrictions that allows us to determine the direction of bias. Second, I show that …

effect having an opposite sign from the true effect. We present an expression for the asymptotic bias of both OLS and IV … estimators and discuss the conditions under which sign reversal may occur. We provide a method for eliminating this bias when …