highly significantP < 0.01
X 2 , X 2 2 , X 1 X 4 had significant effects on EE ( P < 0.05) and X 1 , X 3 , X 4 , X 1 2 , X 3 2 , X 4 2 were highly significant ( P < 0.01) and thus other forms of variables had negligible effects.
X 2 , X 2 2 , X 1 X 4 had significant effects on EE ( P < 0.05) and X 1 , X 3 , X 4 , X 1 2 , X 3 2 , X 4 2 were highly significant ( P < 0.01) and thus other forms of variables had negligible effects.