Vol. 11, No. 7 July 2009
 Contents
 Downturn: Opportunity for quality? Six Sigma FYI: Current releases
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 Software

 Quality Quiz from Professor Cleary Congratulations: "B" is correct. Click here for a more complete video explanation Many people make the mistake of assuming that the Cpk number can be translated into a percentage of capability, but this is not true.  Expressing the meaning of Cpk is a linguistic challenge, but it demands statistical accuracy. Consider the following data for a capability study.  thus USL = 29 LSL = 5 Calculations of Cpk: If Cpk is equal to 1.0, it would be accurate to say that the process is capable of producing at least 99.73% good parts. The percentage is determined to be 99.73 because by calculating plus and minus three sigmas, one captures 99.73% of the area under the normal distribution. (Remember: Z min is divided by three.) What about that vague “at least,” though? If Z usl and Z lsl were both 3, then Z min would be 3 and the Cpk would be 1. On the other hand, if the Z usl is 3 and Z lsl is 5, the Cpk would still be 1, but the process would be capable of producing greater than 99.73% good parts. The lower spec limit will be 5 standard deviations from the mean, producing more than 99.73% good parts.     Copyright 2009 PQ Systems. Please direct questions or problems regarding this web site to the Webmaster.