Cpk 1.99 But Out of Specification Measurement Detected in Control Chart

[fEArmE]

sample = 300
subgroups = 60

Target = 0.093 +/- 0.005"
Some points of 0.98 and 0.99 are seen on charts.

Bad parts detected and it's 0.105"!!!

I'm still scratching my head as how the cpk is still 1.99. I hope someone can shed some light on this. TIA.

[Kales Veggie]

Hi:

For Cpk to be valid, the data has to be normally distributed.

Is the distribution normal? How many out of control points are there? Can you share the actual data with us?

[fEArmE]

I just received the chart from our supplier. 9 points out of specification limit. the x-bar R chart is out of control period.

[Steve Prevette]

Originally Posted by fEArmE

sample = 300
subgroups = 60

Target = 0.093 +/- 0.005"
Some points of 0.98 and 0.99 are seen on charts.

Bad parts detected and it's 0.105"!!!

I'm still scratching my head as how the cpk is still 1.99. I hope someone can shed some light on this. TIA.

Personally, I don't put much faith in cpk. What I suspect is happening is that you have a set of average and control limits that were established at some point in the past. Hopefully during a time when there were no out of control points (1st potential error- doing cpk calculations on a process that is not "in control" is meaningless). Perhaps recently something has introducted a change to the process leading to current out of control points. If they are sufficient to lead you to believe that there is a permanent shift in the process, the average and control limits should be shifted, and a new cpk determined (potential error 2 if this has not been done). If the current out of control points are sporadic, not justifying a shift in control limits, but are indicative of an "out of control process", then again, the cpk value is meaningless as long as you are out of control.

By the way, normality is NOT a requirement to make a control chart. One can argue whether or not normality is needed to make a cpk, but again, I don't put much faith on cpk.

[Kales Veggie]

Originally Posted by fEArmE

I just received the chart from our supplier. 9 points out of specification limit. the x-bar R chart is out of control period.

Then the CpK is not valid. You could also verify their calculation method. It seems unlikely that CpK is high (almost 2.0) with out of control and out of spec points.

[fEArmE]

Thanks for all the reply.

[sea1007]

when you calculate the cpk , i think first you should collect the data and make the X bar - R chart , and based on the result of X-R chart ,make sure there is no instable cause , then you can analysis the cpk .

if there r some date out of limit, what you do is first find the cause and correct it ,and eliminate this data ,confirm the process is stable,then you can calculate the cpk

i just do quality job two years, and this is only my personal thought, welcome to comment
thank you in advance

[David DeLong]

If one has 60 sub-groups of 5 samples on an X bar and R chart and somehow arrives at a Cpk of 1.99, something is wrong.

I would suspect that if you calculated the control limits, some are out of control. Frankly, they would have to be out of control. If that is the case, one cannot calculate Cp or Cpk.

Cpk of about 2 means that 6 estimated standard deviations can fit in the specification limits. How could that happen when some parts are out of spec?

Yup - I would bet my last $$ that you process is not in statistical control.