Why Cpk by itself is not enough

Never draw conclusions based on the Cpk value alone. Learn here how to use a Five-Step Process to Analyze Process Capability:
• Plot the Data as a Control Chart and Interpret
• Construct a Histogram and Compare to Specifications
• Determine the Natural Variation of the Process
• Calculate Cp and Cpk
• Combine All the Info/Summarize Process Capability
At the same time have fun learning from this animated SPC Insights video with Dr. Bill!
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[@stephenpuryear]

The data that you use for deriving "Cp" and Cpk" is itself the result of a measurement process. If that measurement process capability doesn't match the manufacturing process that is paying the bills, you and your customers are headed for heartaches. "Thumbs Up"

[@SPCforExcel]

Yes it does contain the measurement error - far too often that is not considered in talking process capability.

[@stephenpuryear]

@@SPCforExcel I have to agree with you in the strictest sense because all measurements implicitly contain their own errors. The problem is that the size and magnitude of that error can not be eliminated by any mathematical process. This means that measurements presented with no error assessment including their use in charts, can unexpectedly foul up the process control.