Choose a model Nonlinear regression fits a model to your data. You must, therefore, choose a model. With some programs you can express the model as a set of differential equations or as a compartment diagram of boxes and arrows. See Classic equations commonly used by biologists and How models are derived . Also see Why a computer program cannot pick a model for you .
Nonlinear regression is an iterative procedure.
It then adjusts these initial values to improve the fit. See How nonlinear regression works . With Prism, initial values are entirely automatic if you use a built-in equation. If you enter your own equation, you can also enter rules for initial values. For example the initial value of one parameter may be twice the maximum Y value in the data, while the initial value of another parameter may equal the average of the highest and lowest X values.