You'll find it easy to estimate initial values if you have looked at a graph of the data, understand the model, and understand the meaning of all the parameters in the equation. The Initial Values tab of the Nonlinear regression dialog lets you review, and override, the computed initial values. Once you define these rules, Prism will compute appropriate initial values based on the range of your data. 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. If you enter your own equation, you can also enter rules for initial values.

GraphPad Prism provides initial values automatically if you use a built-in equation. It then adjusts these initial values to improve the fit. The program must start with estimated values for each variable that are in the right 'ball park' - say within a factor of five of the actual value. Nonlinear regression is an iterative procedure. Why a computer program cannot pick a model for you.

You must, therefore, choose a model or enter a new model. Nonlinear regression fits a model to your data. Or click Analyze and select from the analyze dialog. If you have replicate Y values at each X value, format the table for entry of replicates.įrom an XY table or graph, click the shortcut button to fit a model with nonlinear regression.

Graphpad prism 8 curve fitting guide