Equation

Intercept

Per capita personal income(t-statistic)

Per capita personal income squared (t-statistic)

Number of counties (t-statistic)

Adjusted R² (F-statistic)

(1)

0.1338 (56.575)^*

0.0021 (17.201)^*

0.134 (295.876)^*

(2)

0.1443 (37.349)^*

0.0006 (1.231)

0.0000401 (3.476)^*

0.139 (154.845)^*

(3)

0.115 (30.854)^*

0.00011 (0.257)

0.0000564 (5.409)^*

0.000509 (20.902)^*

0.300 (272.589)^*