Ok I get it now, thank you both.
If you're curious, I was conducting a repeat of the Franck Hertz experiment with 30 year old equipment. High precision, low accuracy.
I'm putting together a lab report and my result is off from the known value by 1.43 σ. According to the error function tables provided by my prof, and using the error function in my error analysis textbook, that gives me a probability of ~85%.
I don't understand what this means though. Will...