# Potential within Overall Capability Difference?

Six Sigma – iSixSigma Forums Old Forums General Potential within Overall Capability Difference?

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• #50095

Gopinathan
Participant

Hello,
Myself Jayesh , doing graduation in mechanical engineering. I hav used MINITAB for my project. I am confused which Std Deviation shall i use to calcultae Part To Part Variation?? Actually im confused to see TWO Std. Dev values(Within and Overall).
Can anybody explain me… significance of WITHIN and OVERALL, how they are diffrent? there are two Curves WITHIN & OVERALL in capability analysis … Please guide me … Whats Within & Overall is all about????
Thank You!

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#172024

Iain Hastings
Participant

Jayesh,
As you are at a fairly advanced level in your education and you have a copy of Mintab, it should be reasonably straightforward for you to search and use the statguide feature to answer your questions.
No doubt if you are lucky someone will do the cut and paste for you and add it to this thread…

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#172043

Eugene Lanning
Participant

One aspect that is not covered in the stat guide:
I can use CALC to generate 50 normally distributed numbers with std dev of 1 and mean of 0, no challenge there.
Now I select STAT > QUALITY TOOLS > CAPABILITY ANALYSIS > NORMAL and set the USL to 1, the LSL to -1, and the subgroup size to 1.  The result is that the StDev(within) does not equal StDev(overall).   Why, when the subgroup is 1 ( i.e., no grouping), are the two values not equal?
Sorry if this appears to be a trivial matter, but when I asked in the BB training the instructor could not provide an answer beyond “That’s just a quirk of MiniTab”.  For that to be so, in a well used program like MiniTab, is not too plausible.

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#172050

Gopinathan
Participant

Still i didnt get answer to my questions.
I have idea that WITHIN considres SUBGROUP bt how exacly..?
and How Std,.Dev(Within) and std,dev(Overall) are calculated??
Thank You

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#172055

Ropp
Participant

Eugene,
Your instructor was incompetent. You were right to question the answer. Reflects the level of training available I guess.The within and overall calculations are different. The within is simply estimated by the control chart method. So even with subgroup of one, the average moving range (with proper unbiasing factors included) is used to give sd(within). The sd(overall) is calculated by the RMS formula for sd.This answers the original posters questions also.While you can have in some data sets sd(within) larger than sd(overall),in general, within is smaller than overall and is considered to reflect short term variation. It is used to calculate Cpk. Overall, reflecting total variation is used in the Ppk calculation.

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#172058

Mikel
Member

Sample size of 1 uses moving range to ESTIMATE standard deviation.

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#172064

Chris Seider
Participant

Try setting the subgroup size to 50 and see what happens.  And if you have doubt, create a set of 5000 observations and set subgroup size to 5000.
If you do the above, you might understand Stan’s comments.
…………………………………………………..
There are very few “quirks” within Minitab.

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#172080

Eugene Lanning
Participant

An alternative explanation for my instructor is that at that time he just did not have time to provide a better reply — I’ll give hime the benefit of the doubt on that.
Aside from that, with your assistance, I was finally able to find the within clarification in the Stat Guide.   Thanks to all that helped.

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