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

sample 1

(1)

sample 2

(2)

sample 3

(3)

β

0.243513****

(0.0000)

0.214494***

(0.0000)

0.286690***

(0.0000)

γ

9.54E−12***

(0.0000)

1.28E−11***

(0.0000)

8.32E−11***

(0.0000)

N

960

960

584

R²

0.372035

0.28792

0.84293

F-test

283.4850***

193.4752***

1558.995***