Table 2.

Coefficient estimates by the quadratic model

Terms	Coefficient estimate	Sum of squares	DF	Mean square	F value	p value	t-value
X1	0.6294	7.13	1	7.13	16.59	0.0010	4.073
X2	− 0.3517	2.23	1	2.23	5.18	0.0380	− 2.276
X3	1.08	20.82	1	20.82	48.44	< 0.0001	6.960
X4	0.6206	6.93	1	6.93	16.12	0.0011	4.015
X21	− 1.11	3.18	1	3.18	7.39	0.0158	− 2.719
X22	− 0.6176	0.9884	1	0.9884	2.30	0.1502	− 1.516
X23	− 1.10	3.15	1	3.15	7.33	0.0162	− 2.707
X24	− 1.03	2.74	1	2.74	6.36	0.0234	− 2.523
X1X2	− 0.5125	4.20	1	4.20	9.78	0.0069	− 3.127
X1X3	0.5638	5.09	1	5.09	11.83	0.0037	3.439
X1X4	− 0.3575	2.04	1	2.04	4.76	0.0455	− 2.181
X2X3	0.0400	0.0256	1	0.0256	0.0596	0.8105	0.244
X2X4	− 0.1287	0.2652	1	0.2652	0.6170	0.4444	− 0.785
X3X4	− 0.2100	0.7056	1	0.7056	1.64	0.2196	− 1.281