Table 1.

F-ratio and p-value for each independent variable effect in the polynomial response surface models.

Variables		Main effects				Quadratic effects				Interaction effects
		x1	x2	x3	x4	x12	x22	x32	x42	x1x2	x1x3	x1x4	x2x3	x2x4	x3x4
Partition	p-value	0.000 a	0.001 a	_	_	_	0.002 a	_	_	0.001a	_	_	_	_	_
coefficient (Y1)	F-ratio	404.01	225.00	_	_	_	151.29	_	_	104.04	_	_	_	_	_
Purification	p-value	_	_	0.003 a	0.001 a	_	_	0.010 a	0.002 a	_	_	_	_	0.000 a	_
factor (Y2)	F-ratio	_	_	100.80	129.96	_	_	25.50	53.39	_	_	_	_	88.36	_
Yield	p-value	_	0.000 a	_	0.002 a	_	0.001 a	_	0.000 a	0.000 a	0.020 a	0.023 a	_	_	_
(Y3, %)	F-ratio	_	146.06	_	178.22	_	310.81	_	79.03	395.61	17.64	12.78	_	_	_

x₁, x₂, x₃ and x₄: Main effect of PEG molecular mass, TLL, NaCl and pH, respectively. x_{1 2}, x_{2 2}, x_{3 2} and x_{4 2}: Quadratic effect of PEG molecular mass, TLL, NaCl and pH, respectively. x₁x₂: Interaction effect of PEG molecular mass and TLL. x₁x₃: Interaction effect of PEG molecular mass and NaCl. x₁x₄: Interaction effect of PEG molecular mass and pH. x₂x₃: The interaction effect of TLL and NaCl. x₂x₄: Interaction effect of TLL and pH. x₃x₄: Interaction effect of NaCl and pH.

^aSignificant (p < 0.05).