Cpk & Ppk and Differences between Minitab and an Excel Spreadsheet Results

R

rotorsan

#1
#1
I have some measurements from the production data's and analyse it on minitab and on an excel spreadsheet which calculates the Cpk.

As seen on the attachments, Cpk is calculated as 0,99 in excel with formulas but on minitab it is calculated as 1,16 but Ppk is 0,99 - as same as excel sheet-

My collagues in the company uses minitab for their porojects and they choose minitab and calculate Cpk as 1,16. But I put the excel sheet to production and mine is 0,99.

As I am new in statistical controls:(, can someone help me ?

Thanks in advance.
 

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S

silvestri

#2
#2
Hi Rotorsan,
as you can see in the sheet "alternativefigure" I have just added in your Excel file, i calculated it again by hand made formulas and the results are the same. In my my opinion, the spreadsheet you are using is correct.

And, by the graph, you can see you probably would better not validate that process...

I cannot help you much about Minitab, I am just learning it.

Hope this helps. Bye
Gianfranco
 

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S

Sturmkind

#3
#3
Please find the attached. Excel uses formulae for standard deviation that does not use the C4 constant in the denominator to mitigate the bias of the standard deviation estimate. I have attached a screenshot from the Minitab help screen to help assist.

The difference between 0.99 and 1.16 though is pretty small as Cpk follows a Chi-square distribution and these both fit comfortably with standard confidence interval of the mean estimate of Cpk.
 

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Bev D

Bev D

Heretical Statistician
Staff member
Super Moderator
#4
#4
Sturmkind said:
Excel uses formulae for standard deviation that does not use the C4 constant in the denominator to mitigate the bias of the standard deviation estimate.
Click to expand...
Exactly and rightly so.
EXCEL's STDEV function simply calculates the sample standard deviation using n-1 in the denominator. EXCEL can't know what you will do with the standard deviation and they are quite clear on which formula is used for the sample standard deviation.



Of course Sturmkind is also correct in that the difference of a Cpk of .99 or 1.16 isn't enough to get excited about.
 
D

debyang

#5
#5
rotorsan said:
I have some measurements from the production data's and analyse it on minitab and on an excel spreadsheet which calculates the Cpk.

As seen on the attachments, Cpk is calculated as 0,99 in excel with formulas but on minitab it is calculated as 1,16 but Ppk is 0,99 - as same as excel sheet-

My collagues in the company uses minitab for their porojects and they choose minitab and calculate Cpk as 1,16. But I put the excel sheet to production and mine is 0,99.

As I am new in statistical controls:(, can someone help me ?

Thanks in advance.
Click to expand...
Althought Cpk is 0.99 or 1.16 may not be that different, but the calculations in Minitab are correct according to the definitions of Ppk and Cpk defined in AIAG's SPC manual. Because the sigma you used in the spreadsheet is calculated from STDEV function not from Rbar/d2 or Sbar/c4, all the process index you got will be Ppk(unstable process performance index) not Cpk(stable process index).
Minitab graphs tell that the process seems to be stable, but it is possible still some special causes exist leading the overall performance is poor than 1.16, only 0.99.
 
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