In statistics and econometrics, an instrumental variable (IV, or instrument) can be used to produce a consistent estimator of a parameter when the explanatory variables (covariates) are correlated with the error terms. Such correlation can be caused by endogeneity, by omitted covariates, or by measurement errors in the covariates. In this situation, ordinary linear regression produces biased and inconsistent estimates.

呵呵，太坏了。