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While the computations for a process behavior chart are completely general and very robust, the secret to using a process behavior chart effectively lies in the art of rational sampling and rational subgrouping. Rational subgrouping has to do with organizing your data so that the chart will answer the important questions regarding your process.
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Questions asked by average and range charts
A key aspect of the efficient use of the process behavior chart is making the charts answer the right questions. In order to do this, the way the data are organized into subgroups must be matched with the structure present in the data. This usually means that each subgroup will be selected from some small region of space, or time, or product, in order to assure relatively homogeneous conditions within the subgroup.
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Comments
Something I Never Noticed...
Vitally important material, and certainly not universally understood.
In looking at the Ball-Socket data, third organization, though, I noticed something that I had never noticed before, in all the times I had seen Don present this example (and all the times I had presented it myself). There appears to have been a drop from hours one to four, and hours two to five. I had never noticed it before (maybe because the charts in USPC have all 20 hours plotted, not just the first 8 as in these charts). Not saying that anything was actually going on there, but it is consistent across all four cavities (it goes away from hours 9-20 in the full charts).
The signals
Yes, Rip, Hours 1 to 4 are Day One, While Hours 5 to 8 are day two,
and they cleaned the wax off the mold before the start of Day Two.
This wax was the mold-release agent, and its build up during the day,
and occasional removal, was not being attended to systematically
at this time.
Thanks for that clarification!
I do remember the story about the wax...I don't remember its being found looking at the day-to-day. David Chambers was right, I guess...you need to teach this every single day (even to people who have taught it every single day, apparently)!
A Vital Topic Too Often Overlooked
Thanks for another excellent article. Rational subgrouping is either misunderstood or woefully under discussed in the Quality profession.
A google on "Six Sigma
A google on "Six Sigma Rational Subgroup" yields a wealth of incredible confusion and massive misunderstanding of this basic principle. It is time to forget Harry et al, and get back to basics with Shewhart, Deming and Wheeler.
the implicit assumption of figures 6 and 7 that turned
Thanks for excellent article.
About the assumptions I think We don´t assume that the control limits for all cavities at the same time. Each cavity is a different process, so We need separate and calculate for each cavity and then analyse.
Best regards!!
Francisco
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