Any Details of "Difference Charts"?

How to construct "Difference Charts"? :confused:


Sathis

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.

Yes indeed, it is some kind of Short Run SPC chart.

The idea behind this is to chart different products on the same chart even tho have different target.

There are different versions of this kind of chart.
* the difference chart abs(Value-Target_prod)
* the Z chart (or Zed not sure), the you chart all products at once no matter they have different target and variation. The ones that you plot (Value-Target_prod)/Stdev_prod on the same chart


Search for SQCpack. Extracted from their manual, it's not advertizing, just their manual looks OK

In Short-run SPC, special control charts allow you to chart differences from a target value. Sometimes the absolute difference from target is plotted. In other cases, a normalized difference from target is plotted.
The idea is that a zero difference from target is good and any deviation away from zero is bad.
......
A Difference chart is plotting the difference from target in absolute units. A Zed chart takes the difference from target, normalizes this difference in units of sigma, and plots this normalized value as the difference.


:cool:

The attachments may help you.
Giuseppe