Table 3. Significant test for each regression coefficient of the fitted regression model.

Variables	Regression coefficient	Standard error	t-value	P-value	Significance level
Constant	90.9767	0.3564	255.244	<0.0001	**
x1	1.0113	0.2183	4.633	0.006	**
x2	4.4488	0.2183	20.382	<0.0001	**
x3	1.8225	0.2183	8.350	<0.0001	**
x1 2	−0.7596	0.3213	−2.364	0.064
x2 2	−2.7946	0.3213	−8.698	<0.0001	**
x3 2	−1.2721	0.3213	−3.959	0.011	*
x1 x2	−0.3275	0.3087	−1.061	0.337
x1 x3	−0.4200	0.3087	−1.361	0.232
x2 x3	−0.7450	0.3087	−2.414	0.061

*P<0.05, **P<0.01.