SYMPTOM
The Cpk value that I see in the reports is different than the Cpk value that my other stat package shows. Why do the Cpk values not agree?
CAUSE
There are two different standards when it comes to these calculations and QC-CALC is probably not defaulting to the standard you are expecting.
RESOLUTION
There are 2 different sets of standards for these calculations and QC-CALC follows the standards supported by ASQ. The other common standard is AIAG and instead of showing a Cpk value a Ppk value is used. In the Tools – Options – Global – Other section there is a checkbox that will switch to the AIAG standard. Check the Show Ppk instead of Cpk box and you will now see Ppk throughout the program.
MORE INFORMATION
For more information about the differences between the standards and between Cpk and Ppk please see the white paper on this subject.