Help with Capability Assessment

P

puck1263

#1
Hello
Attached is raw data from our process.
A row is a lot....15 data points per lot. Statistical information for each lot is also there.

Current spec is average must be below 5.5
We have not had any failed lots for this requirement.

Customer is considering spec limit of 7.6 with CPk 1.33 for lot to replace current requirement. When I compared the Ppk of the data set with this, Ppk was 1.30...so I thought we would be ok. I looked at Ppk for our current process, by using lot means against the 5.5 average and Ppk was much lower, so I predicted we would be more capable to this spec.

HOWEVER - looking at individual lot performance, if the spec was retro-activated, we would have several lots that would fail......so practically we are not better off. I don't understand. What am I looking at wrong here?

**Note, looking at the data (for capability analysis), you will see that it is neither normally distributed, or in control. Special cause for out of control are known process shifts. Given that stability is a requirement for capability, the predictions had grain of salt attitude, but I did not expect it to be that off.
Also attached are the capability charts of each example above.

Any advice would be appreciated.
 

Attachments

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I

intelbot

#3
Do you have any process adjustment within the lot with Cpk less than 1.33? If there is, the way you are calculating the Cpk is not right.
Or there is none, try to get more samples for the calculation. Use sub-group sample size great than 3, then you will see the curve close to normal distribution. 12 samples is clearly not enough.
 
P

puck1263

#4
intelbot - thanks for your help.
No, no process adjustments for those with under 1.33 Cpk.
Lot sample size is 15.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
Can you give us some idea what the measurement and the process are? That would help us understand the variation you are seing and offer some suggestions.
 
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