When performing nonlinear regression, you don't have to fit each parameter in the equation. Instead, you may fix one or more of the parameters to constant values.

It is often helpful to define constants when you have only a few data points. For example, you might fix the bottom plateau of a sigmoid curve or exponential decay to zero.

Remember that Prism has no "common sense". Prism does not know how you did the experiment. Unless you tell it, Prism doesn't know that a curve has to plateau at zero.

To set constants, click on the Constants button in the Nonlinear Regression Parameters dialog to bring up the Constants dialog. Check the "constant" box in front of the parameters you want to hold constant and enter their values. As a shortcut, you can enter the value first and Prism will automatically select the check box. You cannot enter different constant values for each data set. When you set a parameter to a constant value, you fix it to that value for every data set analyzed.