How to fit different portions of the data to different equations?

In some situations you may wish to fit different models to different portions of your data. This often occurs in kinetic experiments where you add a drug or perform some sort of intervention while recording data. The values collected before the intervention follow a different model than those collected afterwards. Although Prism has no built-in way to fit different equations to different portions of the data, you can achieve that effect using a user-defined equation containing the IF function. For details, see: http://graphpad.com/curvefit/two_models_in_one.htm