Customer is telling me my Cpk calculation is incorrect

ozziegood

Registered Visitor
#1
Hi there!

My customer is telling me that my Cpk calculation is incorrect. This is the formula I am using;

Cpk = MIN(Cpl, Cpu)
Cp = (USL-LSL)/(6* Std. Dev)
Cpl = (Mean - LSL)/(3*Std. Dev)
Cpu = (Usl-Mean)/(3*Std. Dev.)

I am using a 30pc. sample.

He is using the "quality advisor" Cpk calculator. I have put in my information and it does calculate it different.
I have also looked at our Metrology lab's calculation and it is the same as mine.
What am I doing wrong??? HELP!!!:mad:

Thanks:thanks:
 
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M

martin elliott

#2
Re: Being told Cpk calculation is wrong...pls help!

Check the calculation of the Std Dev. you have used. If its EXCEL STDEV then you are calculating Ppk etc.
 
M

martin elliott

#4
Re: Being told Cpk calculation is wrong...pls help!

There are many threads here with explanations of calculation, and even some really good Capability calculators/Excel worksheets.

I have not figured out how to post a redirect :eek: but if you search within this forum you will find what you want.

Hopefully someone else can redirect you and fill in the details.
 

ScottK

Not out of the crisis
Staff member
Super Moderator
#5
Re: Being told Cpk calculation is wrong...pls help!

can you post a spreadsheet of the data and specifications?
I'll run it through Minitab and see what that somes up with...

(and it will help me play with the evaluation some more as I try to talk the boss into buying it for me)
 

Jim Wynne

Staff member
Admin
#7
Re: Being told Cpk calculation is wrong...pls help!

Here is my spread sheet. Please comment and give suggestions. Thanks!
Two suggestions:
  1. Maybe your customer is using STDEVP.
  2. You should be careful about posting files that contain potentially sensitive information. From your spreadsheet I see not only your company's name, but the customer's part description (which includes platform identification). Back when I worked for an automotive injection molding company doing this type of work, we would have loved to have seen what our competitors were up to.
 

ozziegood

Registered Visitor
#8
Re: Being told Cpk calculation is wrong...pls help!

I am looking at the "cpk calcualtor" and it is using the "estimated sigma". So their calcualtion is as follows;

Cpk= Zmin/3

Zupper = (USL-Mean)/Estimated Sigma
Zlower = Mean-LSL)/Estimated Sigma

Estimated Sigma = avg range/d2

What is the d2 for 30 pc and 100 pc??
 

ScottK

Not out of the crisis
Staff member
Super Moderator
#9

Attachments

J

Jim Shelor

#10
Re: Being told Cpk calculation is wrong...pls help!

Discordian,

I ran the spreadsheet data through Minitab and got a different answer.

Can you take a look at my attachment so we can try to figure out what I did wrong?

Thanks,

Jim Shelor
PMP, SSBB
 
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