 When the explanatory variables are not stochastic, then they are strong exogenous for all the parameters.If the independent variable is correlated with the error term in a regression model then the estimate of the regression coefficient in an ordinary least squares (OLS) regression is biased; however if the correlation is not contemporaneous, then the coefficient estimate may still be consistent.There are many methods of correcting the bias, including instrumental variable regression and Heckman selection correction.