Item

p-values and R² of the tested polynomial models

Terms or predictors

Model

p-value

Lack of fit p-value

R²

x₁

x₂

x₃

x₁x₂

x₁x₃

x₂x₃

x₁x₂x₃

Linear

0.0626^ns

0.232^ns

0.43

3.8010*

7.4289***

5.3613***

-

-

-

-

Quadratic

0.0328*

0.564^ns

0.82

3.7098*

5.8522***

3.9040*

1.9614^ns

1.0660^ns

12.2074*

-

Special cubic

0.0624^ns

0.9142^ns

0.91

3.9987***

6.1411***

4.1929***

−1.2882^ns

−2.1836^ns

8.9578*

33.1461^ns