In many situations, especially in the physical and biological sciences, well-known nonlinear equations describe the relationship between variables. For example, pharmacological bioassay experiments can demonstrate how the strength of the response to a drug changes as a function of drug concentration. Sigmoid curves often accurately model response strength as a function of drug concentration. Another example is exponential growth curves, which can model the size of a population over time.

The Fit Curve personality does not require you to specify starting values for parameter estimates or create model formulas. To specify your own starting values and create model formulas, use the more powerful custom Nonlinear personality, which can also fit any nonlinear model. For details, see the “Nonlinear Regression” chapter.