where standard deviation is used?

Question

what is varience?

Answer 1

Real Life example of Standard Daviation

A control chart in manufacturing. We would use them to monitor a process. They can tell you if the process is going, or already is, out of control. The warning and control limits are calculated using standard deviations. For example, the warning limits could be at 2 standard deviations and the control limits at 3. 

I don't know if you want much more detail than that?! In fact, I'm not sure if that's exactly what you were after!