# Capability Rollup

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- This topic has 11 replies, 4 voices, and was last updated 13 years ago by Savage.

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- July 26, 2007 at 7:46 pm #47666
I was talking with a co-worker the other day and he mentioned that in a previous company they used a method of capability rollup. BAsically calculating the capability of subassemblies and them combining them to result in a rolled up, composite capability metric. Kind of like RTY, but using CpK instead. My friend didn’t have much detail about the method. Has anyone done this? Is this a valid method? Any resources for me to research? Just curious – thanks.Matt

0July 26, 2007 at 8:38 pm #159148

M. Salim MSAParticipant@M.-Salim-MSA**Include @M.-Salim-MSA in your post and this person will**

be notified via email.I have done that using Z scores to rollup scores from parts, process, product, software etc into one score for the company ! This is done using scorecards for Z-values and rolled up into one final score ….

0July 26, 2007 at 9:10 pm #159149

Chad TaylorParticipant@Chad-Taylor**Include @Chad-Taylor in your post and this person will**

be notified via email.Why would you want to? Whats the value behind it?

0July 26, 2007 at 9:30 pm #159151That’s what I’m wondering. I can’t imagine a reason for doing this. I am trying to understand if there is some statistical justification for doing this.

0July 26, 2007 at 9:33 pm #159152

Chad TaylorParticipant@Chad-Taylor**Include @Chad-Taylor in your post and this person will**

be notified via email.Matt, there is known. Its stupid

0July 26, 2007 at 9:37 pm #159153

M. Salim MSAParticipant@M.-Salim-MSA**Include @M.-Salim-MSA in your post and this person will**

be notified via email.Stupid for some ….. overall six sigma level for others !

0July 26, 2007 at 9:39 pm #159154

M. Salim MSAParticipant@M.-Salim-MSA**Include @M.-Salim-MSA in your post and this person will**

be notified via email.Key is to have the understanding of a capability score rollup !

0July 27, 2007 at 1:05 am #159164

Chad TaylorParticipant@Chad-Taylor**Include @Chad-Taylor in your post and this person will**

be notified via email.OK, then explain to me how 3 sigma * 3 sigma = 4.1 sigma?????????

Then Explain Why this useful. I have a complete and open mind.0July 28, 2007 at 2:54 am #159223I don’t understand, that’s why I asked. If you do, please share your insight. Thanks.

0July 28, 2007 at 4:32 am #159225

M. Salim MSAParticipant@M.-Salim-MSA**Include @M.-Salim-MSA in your post and this person will**

be notified via email.I can send you some examples if that will help ….. I was referring to the other gentleman when I mentioned “understanding” ! some people are curious which is good …. but some people don’t have the understanding nor do they have the ability to understand !

0July 28, 2007 at 5:12 am #159226

Jim AceParticipant@Jim-Ace**Include @Jim-Ace in your post and this person will**

be notified via email.Matt:

This is how I learned to do this task. From my example you will see why you can not just average the Cpk values or Z scores

Step 1: Find the yield of X independent items.

Example: 5 indpendent items were found to have the following yields;

X1=90%, X2=95%, X3=99%, X4=84% and X5=73%. The 5 items could be the number of steps in a process or the parts in a subassembly.

Step 2: Multiply the 5 independent yields.

Example: .90 x .95 x .99 x .84 x .73 = 0.51904314. This is the rolled throughput yield

Step 3: Find the average yield

Example: 0.519043^(1/5) = 0.877082995. Notice that 0.51904314 = (0.877082995)^5, thus confirming that the average yield is NOT the mean of the item yields.

Step 4: Convert average yield to a Z score

Example: Excel equation =normsinv(0.877083) = 1.160527733

Step 5: Add 1.5 sigma to estimate the average short term capability

Example: 1.16 + 1.50 = 2.66. This is your overall Sigma score for the 5 independent items. Notice that the overall Sigma of 2.66 is NOT the mean of the subordinate sigma scores.

Summary:

Item Yield Sigma (Z)

X1 0.900 2.78

X2 0.950 3.14

X3 0.990 3.83

X4 0.840 2.49

X5 0.730 2.11

Overall 0.877 2.66

Jim Ace0July 29, 2007 at 7:07 pm #159264Thanks Jim. Just before I read your post, I found the same method in one of my reference books. Thanks again.

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