# Control Charts

Six Sigma – iSixSigma Forums Old Forums General Control Charts

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• #48777

nitr
Participant

Is  it possible  to have almost all measurement values inside control limits at S-chart and many points out of  control limits at X-chart;

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#165350

Member

Yes, the S chart is showing the variation within subgroups.  It is easily possible to have a scenario where the variation within subgroups is consistent, but the averages, shown in the x-bar, vary greatly.

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#165357

Ward
Participant

A scenario might be 2 stamping machines performing differently from one another.  You may have consistency within each machine, but not between them.  Your limits on the x-bar chart may be tight (due to the consistency of each machine) but the averages may all fall outside of the control limits.  The number one thing to remember in control charts is “Do not group unlike data on the same control chart”.
Why are you using the x-bar/S instead of the x-bar/R? Once you answer that, I can respond with, “Why do you have subgroups greater than 10?”  and “Why don’t you collect smaller subgroup sizes, (more frequently, if necessary)?

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#165406

Erik L
Participant

nitr,
Another interpretation of this pattern in the control chart, is a non rational subgroup.  Perhaps it could also be pointing to autocorrelated data.  Have you looked at the process data from a Multi-vari/COV/REML perspective?  That should be able to point you to what should be the variance component to key on for the establishment of a rational subgroup.

Regards,Erik

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#165410

Ron
Member

Xbar&R Charts are very flexible and you can subgroup quite large samples (why you would want to I don’t know) when your subgroups are below 25 there is insuffient data to calculate accurate standard deviation calculations.
Xbar&S charts (rarely used) are used when sample size a large 30 40 50 etc.
The interpretation is  the same:  Top portion of the chart displays between subgroup variaiton the lower half within subgroup variation.

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