Why does Prism display "interrupted" rather than results of a curve fit?

Nonlinear regression works iteratively. If you have clean data, reasonable initial values, and an appropriate equation, Prism often converges on a best-fit result in a dozen or so iterations. If not, it may get bogged down in endless and useless iterations. Prism 3 and 4 allow you to control the process:

In version 3.02 or 3.03, you can interrupt fitting after 25 iterations. For a single fit, click "Methods" in the nonlinear regression parameters dialog. Or set the option globally using "Tools...Analysis options".

In version 4, you can specify the maximum number of iterations. Set the option for a single fit under the "Weighting" tab of the nonlinear regression parameters dialog, or globally by choosing "Edit...Preferences..." and then selecting the "Analysis" tab.

When this option is set, Prism will stop calculating and report "interrupted" if it does not reach convergence by the specified number of iterations . This stops runaway curve fits from consuming lots of computer time--especially useful when running nonlinear regression from a Prism script.