Understanding the relationship of Pp, Ppk, Cp, Cpk

C

CONOTX

#1
I'm having trouble understanding the relationship of Pp, Ppk, Cp, Cpk relative to an exercise I'm doing.

With values of:
Pp 0.76
Ppk 0.74
Cp 0.79
Cpk 0.79

It seems that these values represent a product that is off target with high variation, but I'm having trouble with understanding which description fits best.
- Mean shifts between subgroups
- Mean deviation relative to target (this one is my best guess, but I'm still incapable of explaining exactly why)
-Inherent common cause variation (doubt it since Cp and Pp are so close in value)
- No problem at all.
 
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bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
I'm having trouble understanding the relationship of Pp, Ppk, Cp, Cpk relative to an exercise I'm doing.

With values of:
Pp 0.76
Ppk 0.74
Cp 0.79
Cpk 0.79

It seems that these values represent a product that is off target with high variation, but I'm having trouble with understanding which description fits best.
- Mean shifts between subgroups
- Mean deviation relative to target (this one is my best guess, but I'm still incapable of explaining exactly why)
-Inherent common cause variation (doubt it since Cp and Pp are so close in value)
- No problem at all.
If the Cp is similar to the Cpk or the Pp is similar to the Ppk, it generally means the process is close to center. If that exists AND the values are low, then the distribution is likely too wide, although roughly centered. That is only a meaningful estimate if the data is correct (gages used are appropriate, data collection is adequate, minimal sampling error, process being evaluated is normal - and not normal due to the operator overcontroling the process - so that the operator adjustment becomes the process).
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
In addition to the on target (pun intended) comment that Bob made, I would also add that if the Cpk/Ppk Pp/Cp numbers are close as they are then there is very little subgroup to subgroup variation... Ppk also gets larger than Cpk when there is more subgroup to subgroup variation than within subgroup variation.
 
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