The Ppk measure also includes the centering of the process outputs versus specifications. What is Ppk?The Ppk measure provides information on how well a process is actually performing versus the process specifications. result which will generate the higher level of outputs outside of specification. If the mean is (say) closer to the Upper Specification Limit (USL), then we use USL – Mean, to get the worst case result, i.e.

If the mean is centred, either approach gives the same result. Assume worst case.Cpk measures how much a process is in control by measuring its spread / dispersion within the specification limits.Ĭpk = /3σshort Where the Cp and Cpk values are equal, then the process is centered between the specifications, where not equal, then the greater the gap between the two values, the greater the shift in the process mean from the nominal mean. With variation decrease, the process output becomes increasingly homogeneous.Ĭp will normally be used in conjunction with the Cpk measure, so that both centering and spread can be understood. Using Cp alone can therefore be misleading, but it does give a good indication of process potential.Īs the Cp measure increases, the spread of the process output decreases, which is normally seen as positive. The Cp measure does not take into account the centering of the process, so while Cp may indicate a potential to operate within the specifications, due to poor centering, the actual output may be skewed with resultant outputs outside of specification. What is Cp?Cp is a measure of the potential of a process to provide output which is within upper and lower specification limits. Statistical Process Control, Statistical Process Improvement.

Process capability and process control