Cpk 1.99 But Out of Specification Measurement Detected in Control Chart

[Bev D]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

let's restart the thread questions:

we really need the raw data and the spec limits to give you an answer that isnt' a guess.

the other question that I would ask you to ask your supplier is: what FORMULA was used to calculate Cpk? the traditional original formula uses the within sample standard deviation isntead fo the total standard deviation. So the original formula will give you an overestimation of the Cpk value vs what you really want to see. given the PDf you attached - the software is really old (or probably nased on older software) and I woudl suspect the formula first - but again without the raw data it's jsut a guess like any one else's guess.

That said I agree with Steve P - the Cpk thing is a waste. plot your data and look at your data.....that is the best way but it takes work.

 

[Kales Veggie]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

David,

Can you share the raw data with us? If possible put it in a excel spreadsheet and attach. I am very curious about the actual data. A calculation error or incorrect formula is my suspiscion.

 

[fEArmE]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

let's restart the thread questions:

we really need the raw data and the spec limits to give you an answer that isnt' a guess.

the other question that I would ask you to ask your supplier is: what FORMULA was used to calculate Cpk? the traditional original formula uses the within sample standard deviation isntead fo the total standard deviation. So the original formula will give you an overestimation of the Cpk value vs what you really want to see. given the PDf you attached - the software is really old (or probably nased on older software) and I woudl suspect the formula first - but again without the raw data it's jsut a guess like any one else's guess.

That said I agree with Steve P - the Cpk thing is a waste. plot your data and look at your data.....that is the best way but it takes work.

Getting the raw data is impossible. The spec limit is 0.093 +/- 0.005".
The formula used to calculate cpk is cpk=min(cpu; cpl) where cpu=(usl-xbar)/3*sigma, cpl=(xbar-lsl)/3*sigma and estimated sigma is used where sigma=Rbar/d2. I would not suspect the formula. As I mentioned earlier, CP is 3.08 and that represents the process is precise. It's just a very small portion that is out of control and that does not have a big enough impact to cause cpk to be below 1.33 or acceptable level. Correct me if i'm wrong.

 

[Jim Wynne]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

Getting the raw data is impossible.

If that's the case, then someone's going to have to start over if you expect some output that makes sense. Any manipulation you do at this point is based on conjecture and guessing, which is not what you want in a statistical report.

It's just a very small portion that is out of control and that does not have a big enough impact to cause cpk to be below 1.33 or acceptable level. Correct me if i'm wrong.

Before my wife gave birth, just a portion of her was pregnant. Kidding aside, something is either out of control or it isn't. You can't just separate the point(s) beyond the limits (unless you know an anomalous outlier exists) and act like nothing happened.

 

[Miner]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

What type of process is this from? I have seen similar charts from continuous processes such as extrusion. As we discussed in a different thread, some processes are autocorrelated, such as your plastic extrusion process.

When you take consecutive samples from this type of process, your control limits are based on 100% measurement variation. Therefore, any shift in the process will appear out-of-control when, in fact, it is not. The best way to handle this type of process is to use an IMR chart and space your samples apart enough that you are outside the period of autocorrelation.

 

[fEArmE]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

If that's the case, then someone's going to have to start over if you expect some output that makes sense. Any manipulation you do at this point is based on conjecture and guessing, which is not what you want in a statistical report.



Before my wife gave birth, just a portion of her was pregnant. Kidding aside, something is either out of control or it isn't. You can't just separate the point(s) beyond the limits (unless you know an anomalous outlier exists) and act like nothing happened.

jim, thanks for all your constructive comments throughout the discussion. I really dont expect any sensible output when i saw the control chart. I almost fell off of my chair when I see it! I was asking myself how could the quality guy 'qc pass' this part like that!! Not to mention they are iso certified. this is certainly an implementation problem and I know majority of the company is doing the same thing. Trends on charts? what is it?

 

[fEArmE]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

What type of process is this from? I have seen similar charts from continuous processes such as extrusion. As we discussed in a different thread, some processes are autocorrelated, such as your plastic extrusion process.

When you take consecutive samples from this type of process, your control limits are based on 100% measurement variation. Therefore, any shift in the process will appear out-of-control when, in fact, it is not. The best way to handle this type of process is to use an IMR chart and space your samples apart enough that you are outside the period of autocorrelation.

You are right on. It's a plastic extrusion process. Sadly enough, there is no I/MR chart option to choose from.

 

[Miner]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

Are you locked into a subgroup size of 5? If you can go to a subroup size of 2, try taking one measurement then waiting beyond the period of autocorrelation to take the second measurement for your subgroup. Then your control limits will include some short term variation in addition to measurement error. Add a little more time before starting the next subgroup.

 

[fEArmE]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

Are you locked into a subgroup size of 5? If you can go to a subroup size of 2, try taking one measurement then waiting beyond the period of autocorrelation to take the second measurement for your subgroup. Then your control limits will include some short term variation in addition to measurement error. Add a little more time before starting the next subgroup.

I cant answer you right now but will definitely check it out and try what you have recommended. Thanks.

 

[Bev D]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

Getting the raw data is impossible.

That is a problem...and one reason that I am opposed to Cpk. It's the raw data that helps us truly understand. Cpk is so summarized that we can't determine what's wrong when it's bad and there's no way from that single number to even know if it's real when it's good...the challenge at this point is why are you asking for Cpk information? and is this truly meeting your intent?

estimated sigma is used where sigma=Rbar/d2.

Without the data this is just an educated guess but here goes: It's the formula. Rbar/d2 is the within subgroup standard deviation calculation. It ignores the between subgroup variation. The range chart shows small ranges within subgroup but you have large variation between subgroups - Cpk calculated using within sample standard deviation 'effectively takes out' the between subgroup variation. It essentially centers the individual subgroups on the grand average. (Cp effectively centers the subgroups on the target.) That is most likey why the Cpk value is high but there are many individual values out of spec.

It's just a very small portion that is out of control and that does not have a big enough impact to cause cpk to be below 1.33 or acceptable level. Correct me if i'm wrong.

understand that being out of control does not relate to individual pieces being out of spec - these two things are not related. You can be completely out of spec and in control and in out of control and well within spec.

Cpk calculations are not based on control. A statistically controllled process is required for the Cpk value to have any predictive value...