$\Delta D_{it}=\alpha +\beta DE{F}_{it}+\gamma DE{F}_{it}^2+{\epsilon }_{it}$

sample 1 (1)

sample 2 (2)

sample 3 (3)

β

0.106247***

(0.0000)

0.245484***

(0.0000)

0.311585***

(0.0000)

γ

1.55E−12

(0.2817)

−1.19E−12*

(0.7089)

−4.22E−12*

(0.0938)

N

1212

1212

1212

R²

0.085075

0.234589

0.695547

F-test

28.03525***

92.40618***

434.0704****