From all results above, the ln(Pft) may not be endogenous variable because when we use OLS, which means that we treat all right hand site variables as exogenous variables, its coefficient significantly different from zero. However, when it is treated as endogenous variable in IV model, its coefficient turns to insignificant. Moreover, the result of regression-based Hausman test shows that the P-value of ∝_6 is greater than 0.05, so the null hypothesis cannot be rejected, which imply that ln(Pft) may be exogenous variable instead of endogenous variable.