davis007 said:
In my case I have measured the thickness at various points along reels of film. Cutting a wider piece of film into narrow strips produces these reels. Each piece of wide film is a considered a lot of film. When I do a capability analysis of the results I get a different Cp and Cpk depending on which subgrouping I use; Lot, Reel, Point along film at which the thickness is measured. If I use lot or Reel the Cp and Pp values are quite different, indicating that there is Reel to Reel or Lot to Lot variability. When I use point along the film at which the measurement is made as the subgroup the Pp and Cp values agree. So to find out what the potential capability of my process is I need to account for this in between group variability.
This description of your process is very helpful because you have a lot of sources of variation occuring together. You must identify each and choose your rational subgroups carefully.
You will probably have one source of variation that acts across the wide roll that could be caused by such things as non-parallel, tapered, barrel/hourglass-shaped rolls. When you cut the reels into smaller ones they may appear as if they were from different process streams
If you check consecutive linear points in one location, you might see one of two things: 1) pulsating variations from out-of-round rolls, or 2)autocorrelation, where it changes very slowly over time.
If your charts show extremely tight limits around what appears to be normal variation in the average, it could be due to the affect of autocorrelation. In this case you can test for autocorrelation and adjust the time between individual samples to the time when the effect of autocorrelation disappears.
From your process description, I would consider using an I-MR chart with the period between samples set based on the autocorrelation test. For a short time I would also run parallel I-MR charts on both sides and the middle of the roll to verify across the roll consistency.
The charts will provide more useful information than the between/within Ppk/Cpk because you will know exactly where the variation is coming from.