Model term Parameter estimate Degree of freedom Computed t value P value β0 13.87 (5.8) 1     x1 -0.24 (3.9) 1 -0.062 ＜0.001 x2 -0.35 (3.9) 1 -0.090 ＜0.001 x3 0.29 (3.9) 1 0.074 ＜0.001 x1* x2 0.095 (5.1) 1 0.019 0.089 x1* x3 0.094 (5.1) 1 0.018 0.094 x2* x3 0.27 (5.1) 1 0.053 ＜0.001 x12 -0.18 (3.8) 1 -0.047 0.001 x22 0.25 (3.8) 1 0.066 ＜0.001 x32 -0.16 (3.8) 1 -0.042 0.002

Table 9: The least-squares fit and the parameter estimates (significance of regression coefficient)