Calculating Defect Rate from a Cpk value

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Statistical Techniques and 6 Sigma

Calculating Defect Rate from a Cpk value

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Author	Topic: Calculating Defect Rate from a Cpk value
Jim Bongard unregistered	posted 13 January 2001 09:55 PM Is there a simple formula for determining a defect rate (Defective Parts Per Million) for a given Cpk value? Thanks in advance. [email protected] IP: Logged
thebigbear Lurker (<10 Posts) Posts: 4 From:Melbourne Australia Registered: Jan 2001	posted 15 January 2001 10:15 PM Hi Jim, In my search for info on a similar topic (which I posted a request on this site) I've came acrosss an article that may help you. I can't recall the exact title but I have the referance for you. Quality Engineering Journal, Vol 9, No2, Dec 1996, pages 305-316. It discusses the affects of the various types of ditributions on Cpk and the affects on DPPM. It also notes that Skewness and Kurtosis has a significant impact on DPPM. Hope this helps and hope the details are correct. Best of Luck Nick Viney IP: Logged
Marcel Boere Lurker (<10 Posts) Posts: 5 From: Registered: Nov 2000	posted 17 January 2001 03:34 PM Its easy: Cpk is commonly used as a process capability measure for normally distributed populations. The Cpk value is directly related to both the Sigma and the 'distance'of the pop.average to the closest specification limit. Cpk=\|limit-AVG\|/(3*sigma) Thus Cpk=1 --> average 3 sigma from limit. Cpk=1.33 --> average 4 Sigma, andsoforth. In every statistical study book you can find the normal distribution table. Look, in the one side table, for the z(?) value (z in this case equals the number of sigma's you found earlier above). In the table you will find the applicable percentage (or ppm) of the population outside the closest specification limit. Please note you can only use the one sided table if Cpk<< Cp. Other wise repeat the procedure for each limit (upper and lower). IP: Logged

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