Is Cpk a good measure of capability? There are several shortcomings of Cpk

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bobdoering

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For non-normal data, some advocate going ahead with the calculations with no concern for the distribution. Others advocate transforming/manipulating the data into something closer to normal behavior. Which to choose?
Well, for precision machining - neither. Knowing that the distribution is a uniform distribution (if the process has been controlled correctly) from the sawtooth data, the the capability = (USL-LSL)/(UCL-LCL), or the span of the specification over the span of the process variation - which occurs at a constant probability over the control limits. Transforming them to 'normal' is a waste of time and can mask valuable information.

See Statistical process control for precision machining :cool:
 
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