Real Cpk and Static Cpk
Static CpK
Static Cpk is calculated for a feature using the first 25 measurements recorded after that feature is created, or after a selected recalculation point.
Real CpK
Real Cpk is calculated in real time using a feature or process’s most recent 25 measurements. By contrasting this value with Static Cpk, which is calculated from the first 25 measurements, you can learn whether the capability of the feature is improving: capability is rising if Real Cpk is higher than Static Cpk, and falling if Real Cpk is lower. Together, these measures allow you track the progress of your processes over time.
Calculating CpK and Cp
CpK: min(CpU,CpL)
CpU: (USL - μ) / 3σ
CpL: (μ - LSL) / 3σ
Cp: (USL - LSL) / 6σ
USL = upper specification limit
LSL = lower specification limit
σ = process variability (standard deviation)
μ = process mean