How can I plot a one-sided confidence (or prediction) band around a linear regression line or a nonlinear regression curve?

One use for a one-sided confidence band would be on a graph that plots purity of a substance over time. You want to know, with a certain confidence, how low the purity could be at a certain time. You don't really care about quantifying how high the purity could be.

With Prism 3, you can plot the confidence band in only one direction, but it is always the "95%" confidence interval (band). This leaves a 2.5% "error" on each side, so if you only look at one side, this is really a 97.5% interval.

Prism 4 and 5 lets you choose "90%" confidence bands. When you plot in only one direction, this is really a 95% confidence band, which is probably what you want. So you need to make two choices. On the linear or nonlinear regression parameters dialog, choose that you want a 90% confidence or prediction band. On the Format Symbols dialog, choose the best-fit line or curve and make sure that error bars are turned on with the "---" style, but only in one direction.