Why are there no error bars on some points?

Error bar too small

Prism can graph error bars either from replicates you enter, or error values (SD, SEM, etc.) you enter. But sometimes, no error bar appears for certain points on XY graphs. The reason is simple. If the error bar is shorter than the size of the symbol, Prism simply won't draw it, even if the symbol is clear. To see the error bar, make your symbols much smaller. You may also want to switch from SEM error bars (the smallest) to SD or even to error bars that plot the range. 

Bug in Prism 7 (win and mac) for grouped graphs

There is a bug in Prism 7 (up to 7.0b Mac and 7.02 Win) when you enter data into a Grouped table formatted to enter high and low error values, or upper and lower error bar lengths. If one of the error bars is too small to show (given the symbol size), then Prism won't show it. That's fine. The bug is that it also won't show the error bar in the other direction. The bug also occurs if one of the error values is zero, blank or excluded. Prism won't plot the error bar in the other direction.

Workarounds:

• In some cases, you might be able to switch to an XY or Column graph. The bug only affects Grouped graphs.
• Consider plotting raw data (individual replicates) rather than mean and error values. 
• Increase the error value for the error bar that is too small to show until it is just barely able to show. Then Prism will draw the error bar in the other direction.
• Decrease the symbol size so Prism can draw the error values in both directions. 
• Turn off the symbols, and only plot error bars. To do this, set the symbol size to 0. This won't work if one error value is zero, but will work if that value is small.