Table 4.

Results of the augmented Dickey–Fuller test of the time series data.

Region	Data	Dickey–Fuller	p-value
Us	Original	−1.048	0.735
	First-ordered	−1.348	0.607
	Second-ordered	−6.249	0.000⁎
Asia	Original	0.132	0.968
	First-ordered	−1.194	0.676
	Second-ordered	−10.130	0.000⁎
Europe	Original	3.790	1.000
	First-ordered	2.169	0.998
	Second-ordered	−5.538	0.000⁎
Africa	Original	0.199	0.972
	First-ordered	−2.786	0.060
	Second-ordered	−9.139	0.000⁎
South America	Original	3.790	1.000
	First-ordered	2.169	0.998
	Second-ordered	−5.537	0.000⁎
World	Original	−1.182	0.681
	First-ordered	−0.655	0.858
	Second-ordered	−7.234	0.000⁎

^*P$<$0.05 and thus stationary at that level.