6 Replies Latest reply: Aug 10, 2012 2:55 PM by Brian Summ

# CPK Formula

I am trying to write an expression formula to calculate cpk.  In excel the formula looks like this:

"min((USL-mean)/(3*sigma)),((mean-LSL)/(3*sigma))"

Thanks

Brian

• ###### Re: CPK Formula

I too would like help on this.

• ###### Re: CPK Formula

I've never done it before, but I'd suggest to use Statistics Chart Wizard in Tools menu using QV Help guidelines. It allows to operate many of known statistical functions.

Hope it will help.

• ###### Re: CPK Formula

Good info here, Im trying to work on the formula now.

http://en.wikipedia.org/wiki/Process_capability_index

• ###### Re: CPK Formula

Yup, SPC is sometimes challenging.  Attached is a formula that works.

• ###### Re: CPK Formula

What verison of Qilkview are you using? Also, if both my min and max are defined do I need the bottom half the formula?

• ###### Re: CPK Formula

Using ver 11.  But the formula I sent you was written in ver 7.  Don’t really need the second half but our consultant at the time (ver 7) want to be sure to account for missing values.   If I really wanted to be very accurate I would use this formula:  cpk =cp*((1-((ABS(target-mean))/MIN(target-LSL,USL-target))))  This formula is intended to compensate for asymetric specs; eg:  LSL 5  Target 10  USL  17.

Hope this helps.