Why are the error bars on my graph the wrong length?

Prism can generate error bars of various types, and it may be set to show, for example, standard error of the mean when you really want standard deviation, or vice versa.

To change a particular graph, double-click on a symbol or bar. In the resulting format dialog, choose the kind of error bars that you want under "Use as error values" (note that this setting won't show up unless you've given Prism data from which it can compute error bars). If you have more than one data set on the graph, note that the selection is for one data set. Click the ALL button (or hold the control  key)  to make the choice apply for all data sets.

To change Prism's default, go to

Version 2	"Edit... Options... Graph Options"
Version 3	"Edit... Tools... Graph Options"
Version 4	"Edit... Preferences...", then choose the "Graph" tab