Excel spreadsheet shows weird CPK outcome

S

s14realm3

#1
something weird with CPK results on my spreadsheet that I was given.

nomial value- 20.729
tolerance - .50
max spec- 20.66
min spec- 20.16
avg- 20.72
Std Dev- .04
CpK- 4.01
Cp- 4.1

21 pc data sample-
20.751 20.710 20.751 20.694 20.711 20.736 20.729 20.741 20.790 20.730 20.766 20.692 20.753 20.677 20.705 20.759 20.702 20.632 20.654 20.658 20.747

Is the CpK really correct at 4.01?

seems really high. I thought maybe since our tolerance is big (plastic injection) BUT a 4+ CpK? :confused:

Johnny
 
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D

David DeLong

#2
Re: excel spreadsheet weird CPK outcome

Please note that your average is outside the upper spec limit so your program just didn't know how to calculate the Cpk which would be a negative number. Disregard the Cpk or Ppk value.

The Cp (or Pp) could be valid though since that calculation does not take into consideration the average of the process.
 
S

s14realm3

#3
Re: excel spreadsheet weird CPK outcome

thanks for your reply David:

here is another example BUT all the data falls UNDER the lower spec. and I have a CpK of 33.24???

nomial value- 2.37
tolerance - .50
max spec- 2.82
min spec- 2.57
avg- 2.37
Std Dev- .005
CpK- 33.24
Cp- 33.49

2.370 2.370 2.380 2.370 2.370 2.380 2.370 2.380 2.380 2.370 2.380 2.370 2.380 2.370 2.370 2.380 2.370 2.370 2.370 2.370 2.380

is this another case of where my average is outside of the spec. limits so CpK does not count? Is the Cp numbers correct?

thx
 

Stijloor

Staff member
Super Moderator
#4
something weird with CPK results on my spreadsheet that I was given.

nomial value- 20.729
tolerance - .50
max spec- 20.66
min spec- 20.16
avg- 20.72
Std Dev- .04
CpK- 4.01
Cp- 4.1

21 pc data sample-
20.751 20.710 20.751 20.694 20.711 20.736 20.729 20.741 20.790 20.730 20.766 20.692 20.753 20.677 20.705 20.759 20.702 20.632 20.654 20.658 20.747

Is the CpK really correct at 4.01?

seems really high. I thought maybe since our tolerance is big (plastic injection) BUT a 4+ CpK? :confused:

Johnny
First of all...Is your process in control? Otherwise, any number for Cp, Cpk or whatever will be highly suspicious.........:mg:

Stijloor.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
Re: excel spreadsheet weird CPK outcome

is this another case of where my average is outside of the spec. limits so CpK does not count? Is the Cp numbers correct?
Your Cp is close, but your Cpk is wrong, it should be negative. If your program looks at the absolute value, it is wrong. See attached. I rushed it in there, so I may be off a little, but the Cpk is negative, for sure...and that is not a good thing.

Of course, you have no idea what your distribution is, as there is no resolution in your data. In fact, with the tolerance you specify, your resolution is way low. But, that is not your biggest problem today...
 

Attachments

Tim Folkerts

Super Moderator
#6
something weird with CPK results on my spreadsheet that I was given.

nomial value- 20.729
tolerance - .50
max spec- 20.66
min spec- 20.16
avg- 20.72
Std Dev- .04
CpK- 4.01
Cp- 4.1
I'm confused by your specs (and I suspect your spreadsheet is confused too).

With a nominal value of 20.729 and a tolerance of 0.5, it might be reasonable to assume that USL = 20.729 + 0.25 and LSL = 20.729 - 0.25. If you use these values, then the Cpk values do seem to agree agree with the numbers given above.

However, these USL & LSL do not agree with your quotes min and max. I suspect the software is using (Nominal +/- Tolerance/2) to get USL & LSL, rather than using the Min and Max you have listed.


(The same reasoning seems to apply to your second example as well.)


Tim F
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#7
that was my first thought as well. The math for the specs doesn't add up...

if the OP can post the spreadsheet we can provide better advice on the original question.

...of course I'm sure we will all continue to rail against the many drawbacks of Cp/Cpk regardless of the actual original request....
 

jerry_Malaysia

Quite Involved in Discussions
#8
I agree with Stijloor - A process needs to be stable before Cp or Cpk be calculated.
As stated in Juran Quality Handbook "Measuring the inherent or potential process capability requires that the process be stable. Stability is evaluated by a control chart.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#9
I agree with Stijloor - A process needs to be stable before Cp or Cpk be calculated.
As stated in Juran Quality Handbook "Measuring the inherent or potential process capability requires that the process be stable. Stability is evaluated by a control chart.
Some of that comes from the incorrect belief that a process has to exhibit normal (random, independent variation centered about a mean value) variation to be a stable process (false), and then you can calculate capability because the statistics are based on an assumption of normality (true).

Conceptually, capability is a snapshot anyway, and of far less value to describe process variation than people place on it. But, control charts are equally as bad, if prepared and executed incorrectly.
 
R

ralphsulser

#10
Some of that comes from the incorrect belief that a process has to exhibit normal (random, independent variation centered about a mean value) variation to be a stable process (false), and then you can calculate capability because the statistics are based on an assumption of normality (true).

Conceptually, capability is a snapshot anyway, and of far less value to describe process variation than people place on it. But, control charts are equally as bad, if prepared and executed incorrectly.
I agree. Additionally one may want to quantify "how stable is it" and how frequent.
If the mean is not considered then the cp can tell you what to expect as an example to definine tolerence expectations for a process of mating parts. I once used this method to determine Corvette hood flushness when Chevy changed the bonding process specs of the inner to outer skin. I went to a meeting with their chief engineer with my data and it was accepted and revised on the drawing. This was approx. 1978.
 
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