Can CPK be a good manufacturing processor?s quality objective?

D

darkafar

#1
Can CPK be a good manufacturing processor’s quality objective?

My boss chooses CPK to be the manufacturing processor’s quality objective. However I think it’s not a good idea because CPK doesn’t cover variance between groups, and doesn’t reflect the processes’ performance. I intend to tell my boss to use PPK or DPMO instead, but he seems not to know the two indicators.

I want to make sure that CPK is not proper for quality objective, so I’m here for confirmation. Is there someone who uses CPK as quality objective?
 
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Ajit Basrur

Staff member
Admin
#2
Re: Can CPK be a good quality objective?

My boss chooses CPK to be the manufacturing processor’s quality objective. However I think it’s not a good idea because CPK doesn’t cover variance between groups, and doesn’t reflect the processes’ performance. I intend to tell my boss to use PPK or DPMO instead, but he seems not to know the two indicators.

I want to make sure that CPK is not proper for quality objective, so I’m here for confirmation. Is there someone who uses CPK as quality objective?
darkafar,

CpK is a performance index and can be used to ensure QUALITY through a robust and satisfactory process. Thus CpK can be one of the Quality Metrics, and you can have some other parameters (preferably the end result of the process) as Quality Objectives.

Quality Objectives should be SMART (Specific, Measurable, Attainable, Realistic, Timelines). Thus you could have % Rejects, DPPM, % Yields etc as Quality Objectives.
 
D

Duke Okes

#3
Re: Can CPK be a good manufacturing processor’s quality objective?

My boss chooses CPK to be the manufacturing processor’s quality objective. However I think it’s not a good idea because CPK doesn’t cover variance between groups, and doesn’t reflect the processes’ performance. I intend to tell my boss to use PPK or DPMO instead, but he seems not to know the two indicators. I want to make sure that CPK is not proper for quality objective, so I’m here for confirmation. Is there someone who uses CPK as quality objective?
Whether or not Cpk is a good quality objective likely depends on the size of the organizational unit it is being applied to (e.g., not as useful if applied at a macro level for a large unit, versus probably ok if applied to a small unit ... e.g., a specific process) and whether Cpk is a better measure for where the organization is in the quality maturity matrix (e.g., if Cpk will be a good driver of customer satisfaction, profitability, etc.).
 
S

sammy76

#4
Re: Can CPK be a good manufacturing processor’s quality objective?

Ppk is a better indication of the process variation in terms of special and comoon causes of variation.
Usually a Process is first judge with Ppk of 1.67 and if the process passes this criteria than a Cpk study is done with a control chart analysis.
 

Tim Folkerts

Super Moderator
#5
Re: Can CPK be a good manufacturing processor’s quality objective?

My boss chooses CPK to be the manufacturing processor’s quality objective. However I think it’s not a good idea because CPK doesn’t cover variance between groups, and doesn’t reflect the processes’ performance. I intend to tell my boss to use PPK or DPMO instead, but he seems not to know the two indicators.

I want to make sure that CPK is not proper for quality objective, so I’m here for confirmation. Is there someone who uses CPK as quality objective?
I think a big part of the problem is that Cpk has much more name recognition that Ppk. From what I have heard around the Cove here, it isn't unusual for people to say Cpk, but actually calculate Ppk. If you really want to understand your process, you really ought to look at Cpk, Cp, Ppk AND Pp. At one extreme is Cp, which tells you how well you could do if the process was constent and centered. At the other extreme is PPK, which tells you how well your are actually doing, given that the process isn't completely consistent ande isn't perfectly centered. Cpk & Pp fall inbetween.

I agree with Sammy76 that the single best indicator of the process is Ppk.


I have two competing opinions about DPMO. At one level it simply tells the fraction of defects found - count the defects and divide by the number of opportunities. You could just as well express this as DPHO (per Hundred, ie percentage) - no difference other than the location of the decimal place. Either form is an excellent indicator of performance.

My problem comes when people try to extrapolate the data to predict DPMO. They start with imprecise estimates of mean and standard deviation, make questionable assumptions about normal distribution, then proceed to predict the defect rate to 2 or 3 digits. In many cases I wouldn't trust these calculations to within an order of magnitude, let alone distinguishing between 3.4 DPMO and 3.5 DMPO.


As for your boss, I'm not sure how to convince him to use Ppk rather than Cpk. Perhaps you could subtly lead him toward the conclusion you want. Ask him what it is that he really wants to measure - the "hypothetically" :( possible capability or the "actual" :) capability. Or ask how he wants to calculate the standard deviation - using an "estimate" :( where you "have to" :( average together a bunch of individual calculations, or "directly" :) calculating the standard deviation from all the available data. Just like a good magician, you can probably force him into a specific response while making him think he was acting freely. :notme: Then tell him that what he chose is really known as Ppk rather than Cpk.

Or just keep reporting the Cpk that he asked for and the Ppk (and perhaps Cp as well) that you think are important, and interpret what these two number tell you. He can always prune out the Ppk value if he really wants, but at least you have given him the numbers that he really should have.


Tim F
 
Y

Yew Jin

#6
Re: Can CPK be a good manufacturing processor’s quality objective?

Please make sure the process is in the statistical control before we study any process capability. In statistical control means that the process must be stable and capable through control chart plotting. :cool:
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#7
Re: Can CPK be a good manufacturing processor’s quality objective?

An additional thought - Cpk relates to customer specifications. If the specifications change, the Cpk changes. How do we know that the customer specs are correct? And - is mearly making a part to customer specs sufficient? We don't capture innovation - manufactureres of carburetors went of of business not because of not meeting specs, but because something came along that replaced carburetors. Also, elimination of defects (which is basically what Cpk relates to, though it does include variance reduction) does not necessarily leave you with a good product. Cpk also causes you to focus upon the numerical targets - and this is also an issue with Six Sigma.
 

ScottK

Not out of the crisis
Staff member
Super Moderator
#8
Re: Can CPK be a good manufacturing processor’s quality objective?

Is there a definitive thread on the differences between and uses for the various process capability and performance indexes?
 

Tim Folkerts

Super Moderator
#9
Re: Can CPK be a good manufacturing processor’s quality objective?

Is there a definitive thread on the differences between and uses for the various process capability and performance indexes?
This thread is pretty good fopr explaining the various indices: http://elsmar.com/Forums/showthread.php?t=1994

I was thinking about trying to write an article explaining the various capabiltiy indices. My thought was to start with run charts of various sets of hypothetical data that illustrate particular good and bad features in data. After showing if the data was in control, I would calculate the various indices and discuss how and why they are different. I haven't seen such an approach and I think it might be a good way to get more of a gut feeling for the indices. If this sounds interesting/valuable to people here, it might spur me to make this a little higher priority. :yes:


Tim F
 
D

DsqrdDGD909

#10
Re: Can CPK be a good manufacturing processor’s quality objective?

I for one would be very interested and might send the author of such an article some good cigars or golf balls.
 
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