Are my Cpk and Cp Calculations Correct?

J

jonnyollet

#1
Dear All,

I have been reading through the forums about how to construct Cpk and Cp calculations in Excel.

I have completed a spreadsheet, but not sure if the values for Cpk and Cp I am getting are correct.

For this particular product, we are only measuring one critical dimension, which we need to be in control.

I have used the MR bar over 1.128 to calculate the Standard Deviation and used this for my Cpk and Cp calculations.

If anyone has any corrections I need to make, please can you let me know. I am new to this, so please be kind.

Many thanks
 

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Stijloor

Staff member
Super Moderator
#2
Dear All,

I have been reading through the forums about how to construct Cpk and Cp calculations in Excel.

I have completed a spreadsheet, but not sure if the values for Cpk and Cp I am getting are correct.

For this particular product, we are only measuring one critical dimension, which we need to be in control.

I have used the MR bar over 1.128 to calculate the Standard Deviation and used this for my Cpk and Cp calculations.

If anyone has any corrections I need to make, please can you let me know. I am new to this, so please be kind.

Many thanks
Can someone provide comments/feedback?

Thank you!

Stijloor.
 
D

Darius

#3
Well, chart is not a control chart but a run chart with specs limits attached.

The formula looks ok for IxMR but with one little error: the range for MR average does not cover all data range.

note:It could be an improvement to calculate control limits and add an histogram
 
Last edited by a moderator:

Bev D

Heretical Statistician
Staff member
Super Moderator
#4
the formula for converting the moving range to an estimate of the standard deviation is correct. however, for Cpk you should use the total standard deviation direct calculation using STDEV function. MR bar/1.128 is an estimate of the within subgroup variation. that is appropriate for Cp. Cpk* is intended to be current actual capability which must include all of the variation (within subgroup and between subgroup).


*I'm assuming that you are the more popular terms for the capability indexes which tend to mean Cp = Cp and Cpk = Ppk.

I will also point out that your process is not Normally distributed so the Cpk value wont' be all that insightful except as a rough indicator that you are not capable (which the simple run chart against the spec limitis shows you without any fancy math). This could be even more confusing if you translated that Cpk value into a defect rate... Additionally there are some who will point out that your process is not in statistical control and therefore it is inappropriate to calculate capability indexes but I don't knwo why you are calculating the indices so it may be irrelevant. Plus - you only aske dif the formulas were correct. :)
 
Last edited:

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
Attached are the results you can benchmark your results to from QI Macros. As BevD suggested, they use STDEV for the standard deviation, also.
 

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