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Quality Control Index - We thought that Cpk or Ppk would be good choices

V

vskiller

#1
Hi
Our manager have asked us to work out a quality control index. We thought that Cpk or Ppk would be good choices, but some problems arose!
1. I have searched in the Cove and could find some articles discussing differences between Cp, Cpk, Pp and Ppk, but they were not so clear. In some cases, it is said that we have to collect more that 60 samples. But our company is producing just 10-20 products a year!!! and we have about 40 collected data.
2. As mentioned above, we are producing 10-20 products a year. Therefore, we have actual sigma, not sample sigma. then, we can't use Cp/Cpk indices. Is this true?
3. Management needs one and only one number as the measure of quality. Can we summarize Ppk? (normal summation or average, weighted sum, products of Ppks, etc.)
Thanks
 
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Statistical Steven

Statistician
Staff member
Super Moderator
#2
Hi
Our manager have asked us to work out a quality control index. We thought that Cpk or Ppk would be good choices, but some problems arose!
1. I have searched in the Cove and could find some articles discussing differences between Cp, Cpk, Pp and Ppk, but they were not so clear. In some cases, it is said that we have to collect more that 60 samples. But our company is producing just 10-20 products a year!!! and we have about 40 collected data.
2. As mentioned above, we are producing 10-20 products a year. Therefore, we have actual sigma, not sample sigma. then, we can't use Cp/Cpk indices. Is this true?
3. Management needs one and only one number as the measure of quality. Can we summarize Ppk? (normal summation or average, weighted sum, products of Ppks, etc.)
Thanks
The question is why....why do they need a single number of quality with such a small population of products. I assume that every unit gets inspected, and with just 10-20 products produced a year, the capability index might not have such great value.

Having said that, the difference between Cpk and Ppk is the estimate of sigma. Since you have a sigma off all the items (population per se), you can use this value and call it Qpk or Quality Index. What you call the index is less important than how it will be used to improve quality.
 
P

preetham - 2007

#3
Statistical process study will be done on process to see that the process is capable of producing the parts without any major variation .

you are telling that you are producing 12-25 parts per year i don't see any necessity of doing the SPC study as you will be checking all the parameters of the part. the SPc study will be done on the process where you can not do 100% inspection.
 
V

vskiller

#4
All of us know managers, don't us?! All managers I know, want to decide about their firm with a few data. My current manager too! Also, I have to mention that our products have more than 1000 parts... Then there is not a few measures and factors. We have a few products a year, but lots of factors. :frust: Therefore, we really need to minimize the number of our decision factors, especially some critical ones.
We just thought that Cpk or Ppk would be good indices, no more! So, if you have any suggestions, we would be pleased to hear from you.

Thanks
 

Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#5
Welcome vskiller! :bigwave:

Cpk is a process capability measure that is statistically created, usually with many parts ongoing. It only measures a process's ability to consistently produce a part. It doesn't measure quality or the impacts of things done well.

How about if you measure something having more to do with quality, like cost of poor quality, scrap rate, improvements in cycle time and customer satisfaction?
 
V

vskiller

#6
Hi Big boss! :cool:
I think cost of poor quality evaluation is too hard! And I have no idea how to do that.
And we have some other problems too!
first of all, my manager is really interested in Statistical Quality Control and mainly in six sigma (He himself is not sure whether SS is applicable to our company or not).
second, our company pays extra money for more efficiency (merit pay maybe!), now the CEO forced us to implement an index that reflects the efficiency and quality level of company, and future payments be based on this index!
Third, We want to improve quality and we do not know how to start!!!

Then! I really need your help guys!
Thanks
 
A

Angusk

#7
HI VSKILLER

If I understand correct, you are an assembly company.

My company produces up to 20 different parts a month and we do capabilty not on the number of samples we take. we requested because of the variance in parts and inconsistency will occur we will take our capability on the amount of readings we took, so we could take five readings off 1 part which is a diameter for x amount of parts, but what you could attempt is to do Cpk's on the sub-parts, If you give me more info on the inputs I would be able to help you further.

Just remember there is never a boundry with quality
 
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