# Cpk vs Ppk

• This topic has 6 replies, 7 voices, and was last updated 14 years ago by Ron.
Viewing 7 posts - 1 through 7 (of 7 total)
• Author
Posts
• #50005

Dan B
Participant

Can somebody explain, in plain language, the difference between Cpk and Ppk?

0
#171780

Cao Rongshun
Participant

Cpk is for short term, Ppk is for long term.

0
#171783

Taylor
Participant

CpK-Short term capability
PpK-Long Term Capability

0
#171784

annon
Participant

Cpk and Ppk are both estimates of process capability.  Capability is essentially the porportion between the variability allowed by the customer (spec limits) and the variability in your process (std dev), and since the spec limits are given to us by our customer, the only thing we can change in this context is the type of std dev used in the calculation.
Cpk uses short term estimates, those that are said to be devoid of special cause.  Since our process is rarely devoid of special cause, it is often useful to use long term std dev, which is gathered over a longer period of time and includes both between and within variabililty estimates…it relates more to actual process performance.
Cpk is something of an idealic state, while Ppk is more represenative of what your customer is experiencing, it is their reality.

0
#171785

Kannan
Participant

i second annon. Just to add. Process capability analysis entails comparing the performance of a process against its specifications. We say that a process is capable if virtually all of the possible variable values fall within the specification limits. Cpk assumes that the data  is normally distributed. The calc for Cp= ((USL-LSL)/6s).
We can modify this equation slightly to account for off-center processes to obtain the Cpk index = min( ((USL-x)/3s), ((x-LSL)/3s) )

you can learn this graohically @  http://elsmar.com/Cp_vs_Cpk.html

0
#171848

melvin
Participant

I believe that the difference is in the way sigma is calculated.  Cpk uses sigma estimate based on Rbar/d2 for control chart.    Ppk uses all the individual data to calculate sigma using the standard formula for sigma.  After that difference, the calculation of both is the same.  Hope that helps a little.

0
#171849

Ron
Member

Cpk is short term PPk long term

one sample versus 25 samples

0
Viewing 7 posts - 1 through 7 (of 7 total)

The forum ‘General’ is closed to new topics and replies.