Does anyone know the conception of "Plant Cpk average"?

[lydielu]

Dear all, i have a question for you. does anyone hear this:
CPK Averages
Individual CPK part values must be combined and averaged to report a “Plant CPK Average” at each plant.
The Plant CPK average will be reported on a monthly basis.

i always know apply Cpk or Ppk to one process, for statistically controlling the processs of manufacturing some part, so, what the plant Cpk means and for what to do this?

[Wes Bucey]

I presume this is a request from a current or prospective customer. It appears from the request the folks making the request want a simple measuring stick to determine if the entire plant is running efficiently.

I don't necessarily agree with the concept, but it seems simple enough to do the arithmetic if your plant is already calculating Cpk for each individual part you manufacture.

For those who may like (or need) a quick refresher in Process Capability and Process Performance, you might start here for a clear definition of each:
http://www.isixsigma.com/library/content/c010806a.asp

[Claes Gefvenberg]

Originally Posted by Wes Bucey

I don't necessarily agree with the concept

Nor do I... It sounds a bit strange imo. Maybe I'm dense, but I can't really see the use of such a figure? Anyone else? Where is Steve when we need him?

/Claes

[Wes Bucey]

Originally Posted by Claes Gefvenberg

Nor do I... It sounds a bit strange imo. Maybe I'm dense, but I can't really see the use of such a figure? Anyone else? Where is Steve when we need him?

/Claes

Actually, one could infer a lot of things from such a number:

1. Do the folks at this plant KNOW how to calculate a Cpk?
2. If the number seems too "pat," we can ask for the raw data - folks have been known to "fudge" reports, but it takes a real expert to fudge raw data by working in reverse from a phony Cpk.
3. This may be a test of Supplier Responsiveness - will they comply with a relatively simple request?
4. If the number reported is between 1.33 and 2.0, we might conclude the supplier is using adequate equipment and processes. If the number is much higher, we might conclude the capability is much higher than current customer requirements - we would wonder why - do they pursue scut work with wide open tolerances? do they really report a factual number or is it "created" to look good? In any case, we would investigate in more depth.
5. If the number is near or below 1.0, we would seriously wonder why these folks were in business with machines or processes that can't maintain control for the tolerances required by customers.

[Claes Gefvenberg]

Originally Posted by Wes Bucey

Actually, one could infer a lot of things from such a number:

As you say, but one would have to look deeper to understand what this average figure actually is worth.

It's just that I could think of easier ways of finding out whether the supplier knows how to calculate a Cpk or not, and I would much rather like to see individual Cpk figures. This average could be pushed up, for instance by throwing in results like you suggest in no. 4, and a low average could mean that some tolerances are nigh on impossible to live up to with the best available equipment.

A test of Supplier Responsiveness? Perhaps, but why not test that with something more useful?

Anyway, has anyone else come across this?

/Claes

[Wes Bucey]

Originally Posted by Claes Gefvenberg

As you say, but one would have to look deeper to understand what this average figure actually is worth.

It's just that I could think of easier ways of finding out whether the supplier knows how to calculate a Cpk or not, and I would much rather like to see individual Cpk figures. This average could be pushed up, for instance by throwing in results like you suggest in no. 4, and a low average could mean that some tolerances are nigh on impossible to live up to with the best available equipment.

A test of Supplier Responsiveness? Perhaps, but why not test that with something more useful?

Anyway, has anyone else come across this?

/Claes

We have both commented it is NOT the way we would do things. That said, a lot of folks have their own quirky "rules of thumb" in which they have a lot of faith. One lady known to me has a way of checking out a new restaurant which may seem bizarre, but she swears by it:
Before allowing herself to be seated, she goes to the restroom (toilet) and looks in. If it isn't spotless, she leaves.
Obviously, she is not from New York City, where restaurant bathroom facilities can be pretty harrowing even in some otherwise nice-seeming places. She would NEVER eat in a restaurant there.

[buffalo_hua]

Hi lydielu
I am not sure it was proposed by your customer? if yes, let it alone just like nonsense, the so-called new conception must be concocted by your customer, as far as know, we can only calculate the monthly CPK for each preocess and then take the relevant measures to improve the processes to achieve the goal of >1.33.
FORGET IT!!

[Tim Folkerts]

Whether or not you like the idea, there is also a statistical question as to just how to average.

As an example, suppose you you produce 1,000,000 pieces of Part A on 10 production lines each with a Cpk of 2, and 1,000 pieces of Part B on one production line with a Cpk of 1.

The average of the two types of part is clearly 1.5.

The average of the 11 production lines is 1.9

The average of the 1,001,000 parts is very close to 2.


Clearly, this could get complicated. You and the customer would both have to know what method you used. Or just use the method that gives the best looking result and make the customer ask if they want details

Tim F


P.S. This reminds me of an old statistics puzzle. Two competing suppliers - Vendor X and Vendor Y - produce the same set of products: Part A, Part B, Part C, .... Vendor X claims that his factory produces fewer ppm defective overall. Vendor Y claims that his factory makes each and every part with fewer ppm defective. Both are correct!