This example requires a User-defined model in **Regress+**. The model, typed into a dialog (or cut-and-pasted from a textfile library), is parsed, saved and thereafter interpreted.

This example also illustrates one of the pitfalls of mathematical modeling. Looking at the formula, one wonders, "Why not the simpler, algebraically equivalent model shown below?"

y = 2.661 (1 − exp(−1.484 (x − 1.040)))^2.796 − 16.27

The reason is that, for x < 1.040, the latter requires raising a negative number to an arbitrary power, giving an imaginary result.