SPC Charting - Calculating Cpk & Ppk

[Former Member]

Hi all,

Let me preface this by saying that I am not an SPC expert...I know the fundamentals. With that being said, can anyone explain why UCL and LCL limits have to have 'Auto-Calculate' checked in order for Cpk/Ppk to be calculated correctly. I know you need these values (along with USL and LSL) - but why do the control limits have to be auto calculated?

I have a client that has fixed control limit values (based on previous year) - so, I don't want 'Auto-Calculate' enabled in their pre-defined charts.

Any insight is greatly appreciated.

Joseph Scott

[jcgood25]

Without the auto-calc have you looked at the EST_XXX summary stats such as: EST_CP / EST_CPK ?

[Former Member]

Hi Jeremy,

Yes. I am OK with the Ppk as this and EST_CPK are equivalent - so, I always get correct values for this.

But, for Cpk - I can only get the correct value with auto-calc checked. None of the EST_XXX statistics match this value...

So, users have to generate chart w/ auto-calc off - to see if any points outside of control limits. Then chart has to be re-generated w/ auto-calc on - to get proper Cpk value.

Thanks