There will always be differences in people’s performance. The question is, are the differences real? If the people work in a process then most of the differences will be caused by common-cause variation. Control charts are the only way to establish if the differences are attributable to individual differences.

If you do establish a difference on control charts, then you still have to decide if the difference is not due to causes outside of the individual’s control. When you have discounted all other possible causes of variation, you can assume you are dealing with individual differences.