Kevin,
Good thoughts and not out on a limb at all. I have made several assumptions in my responses to this thread and you know what happens when you assume. Anyway:
Coming up. : >)
Is a two sample subgroup sensitive enough…
I believe you are thinking of an X-MR chart. The Moving Average (MA-MR) chart I mentioned is normally comprised of moving subgroups of three, which would give eleven data points to chart (I can expound upon this later if you like). This type of chart is not as sensitive as the typical Xbar-R charts, but are useable if you know the limitations. I suggested it as a mechanism that
could (not should) be used to demonstrate control, if such a demonstration were required.
I’m thinking that you might be better off…
For this particular case, there are several methods, your suggestion being one, under the assumption (again) the data was available. From what I have assumed so far, I thought the process was not running and the twelve pieces were all that were available.
The one thing that keeps popping up…has been compromised
I agree. My head too. I (personally) have made it a practice to try to not perform capability studies unless I was sure (certain within a certain C.I.) of the integrity of the samples taken and the statistical control of the process. For a continuous process, the sample size should be based on tolerable error and confidence level desired, in addition to what I have pointed out earlier.
For example, standard deviation is estimated at 0.002, tolerable error is 0.001 and I want to be 95% confident in my results. NOTE: Standard deviation and tolerable error must be the same unit of measure, i.e. inches, meters, pounds, stones, kilos, electron volts, light years, warp factor, etc. Sample size is then n = [(1.96 * 0.002)/0.001]^2 or 15.37, rounded up to 16 (1.96 is from the Z table for 95% confidence).
The other tidbit of information needed would be is this a formal study (my customer is going to see and examine it) or an informal (I just would like to know). If the former, be prepared to demonstrate to the customer’s satisfaction the study is accurate and correctly performed. If the latter, the rules can be relaxed as much or as little as personal comfort level allows. I have relaxed (broken?) more of these rules than I care to recall. I am not a statistical purist, and hope I do not appear an ( o ) on the subject.
If I examine a study that purported to prove a process is capable, you can bet I am going to ask those types of questions. If I were the customer, your answers to those questions would determine if I accepted the study or not.
I have rambled enough. Does that answer your questions (concerns)? If not, lemme know.
Regards,
Don