How can I fit a three-phase exponential decay curve to my data?

Enter this user-defined equation:

Y=Span1*exp(-K1*X) + Span2*exp(-K2*X) + Span3*exp(-K3*X) + Plateau

You may be able to omit the "Plateau" variable if you know that the curve must plateau at zero. You will need to enter initial values. Rather than rules, which are more useful in the long run but harder to set up, just enter values for the parameters that seem right based on inspection of the data. You can try them out first. From the nonlinear regression parameters dialog,

• Prism 3: Click "Initial values...". Check the box "Don't fit. Fix all variables to their initial values."
• Prism 4: Set the radio button labeled "Don't fit. (Plot the curve defined by the initial values.)"

Prism will just draw the curve based on those initial values. If the curve is near your data, then go back ("Change... Analysis Parameters") and tell Prism to do the fit. Otherwise, tweak the initial values.