For a quantitative factor, use orthogonal polynomials (Section 2.3) to further model the main effect of the factor. First- and second-degree polynomials are commonly used. Fourth- and higher-degree...

For a quantitative factor, use orthogonal polynomials (Section 2.3) to further

model the main effect of the factor. First- and second-degree polynomials are

commonly used. Fourth- and higher-degree polynomials are rarely used because

of the problems associated with overfitting and interpretation.