Cp vs. Cpk - A Visual Animation and Related Discussion Threads

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ScottK

Not out of the crisis
Staff member
Super Moderator
#2
Thank you Marc.

That will help with around my office with a lot of misconceptions.

Is there one for measures of central tendency? We're being hit pretty hard right now by a customer who wants minimum Z value of 4.5 on certain variables.
 

Tim Folkerts

Super Moderator
#3
I really like the animations. I think they are a geat way to get a feel for the difference between Cp & Cpk, and alos to show how the values change as the process changes.

How did you create these animations? I have been thinking about doing such animations myself for a couple different projects?


One critique: on the bottom animation, shouldn't it really say "Capable/Not Capable" rather than "In Control/Out of Control"? Whether or not the process is outside the spec limits, the process can still be in control (or out of control). A control chart - not a histogram or Cpk calculation - would be the tool to determine control.


Tim F
 

Manix

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#4
Thanks Marc,

These are great! :applause:

One critique: on the bottom animation, shouldn't it really say "Capable/Not Capable" rather than "In Control/Out of Control"? Whether or not the process is outside the spec limits, the process can still be in control (or out of control). A control chart - not a histogram or Cpk calculation - would be the tool to determine control.


Tim F
I agree with this point.
 
A

Ashok GS

#5
Cp and Cpk are tricky to understand and such animations helps to grasp the concepts so quickly.

Great work and thanks for sharing ! :applause:
 
U

Umang Vidyarthi

#6
Hi Marc,

Good you posted it here. I saw it within a week of joining the cove,on the forum, it is at the bottom of second column from the left. Some responses here indicate that people have missed this great presentation, which of course is not un-usual. The benefit is that the discussion here is bringing out other opinions, like the one pointed out by Tim, which appears valid. It will be intresting to see other opinions as well.

Umang :D
 

Statistical Steven

Statistician
Staff member
Super Moderator
#7
The animation assumes a centered process. The ratio of Cpk to Cp gives a rough estimate of how well the process is centered. A ratio of 1 means that the process is perfectly centered. You can have a Cp of 5 with a Cpk of 0, so in reality they are independent measures.
 

reynald

Quite Involved in Discussions
#9
For analysis yes because it would be very meaningful to compare them.
Case 1: Cp = 1.5, Cpk = 0.98 --> your problem is centering
Case 2: Cp = 0.98, Cpk = 0.98 --> your process is perfectly centered, your problem is variation
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#10
:deadhorse: and of course, a simple look at the data will tell you the same thing and a whole lot more. no need to go through the calculations (unless of course your customers are requiring the numbers.)
 
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