Re: What is Difference Charts?
I did not recognize it by this name, but Google shows that it is a type of Short Run SPC where the difference between the sample measurement and a target value is plotted on the control chart.
For example, you have a length cut-off process that consists of a lot of short production runs at hundreds of different target lengths. The variation is the same regardless of the target setting. The center line of the X-bar or X chart is zero. The R-bar or MR-bar is calculated the standard way. The control limits are calculated using the standard method (using R-bar or MR-bar), but are applied about the zero target line.