CHAPTER 15 Introducing Correlation and Regression 211

The predictor variables can appear in a formula in nonlinear form, like squared or

cubed, inside functions like Log and Sin, and multiplied by each other. But as long

as the coefficients appear only in a linear way, the function is still considered lin-

ear in the parameters. By that, we mean each coefficient is multiplied by a term

involving predictor variables, with the terms added together in a linear equation.

A nonlinear function is anything that’s not a linear function. For example:

Y

a

b

e ^c

X

/

–

is nonlinear in the parameters, because the parameter b is in the denominator of

a fraction, and the parameter c is in an exponent. The parameter a appears in a

linear form, but if any of the parameters appear in a nonlinear way, the function

is said to be nonlinear in the parameters. Nonlinear regressions are covered in

Chapter 19.