What does a Process Capability Index of greater than 1 suggest?

G

goallout - 2009

#1
Can anyone help me in answering this question.
"Assuming that you know its standard deviation, explain how the capability of a process to deliver product quality can be determined & explain what a Process Capability index of greater than 1 suggests."

I am stuck with how to answer this question in my course study as I have miss some of my class lessons

Thanks
 
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Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#2
There some analogies in the iSixSigma web site. Ken K's is:

"The value itself can be thought of as the amount the process (car) can widen before hitting the nearest spec limit (garage door edge).
Cpk=1/2 means you've crunched nearest the door edge (ouch!)
Cpk=1 means you're just touching the nearest edge
Cpk=2 means your width can grow 2 times before touching
Cpk=3 means your width can grow 3 times before touching"
To explain, Ben says:
Consider a car and a garage. The garage defines the specification limits; the car defines the output of the process.

If the car is only a little bit smaller than the garage, you had better park it right in the middle of the garage (center of the specification) if you want to get all of the car in the garage.

If the car is wider than the garage, it does not matter if you have it centered; it will not fit.

If the car is a lot smaller than the garage (six sigma process), it doesn't matter if you park it exactly in the middle; it will fit and you have plenty of room on either side.

If you have a process that is in control and with little variation, you should be able to park the car easily within the garage and thus meet customer requirements. Cpk tells you the relationship between the size of the car, the size of the garage and how far away from the middle of the garage you parked the car.
 

Stijloor

Staff member
Super Moderator
#3
Can anyone help me in answering this question.
"Assuming that you know its standard deviation, explain how the capability of a process to deliver product quality can be determined & explain what a Process Capability index of greater than 1 suggests."

I am stuck with how to answer this question in my course study as I have miss some of my class lessons

Thanks
Hello goallout,

This may be of help: "Cpk Vs. ppm Table."

Stijloor.
 
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