Spc Process Capability

SPC Process Capability is the statistical measure of how well a process meets its specifications — comparing the process spread (variation) to the specification range, quantified by indices like Cp, Cpk, Pp, and Ppk that indicate whether the process is capable of consistently producing within-spec output.

Capability Assessment

• Data Collection: Measure the quality characteristic on a representative sample — typically 50-100+ measurements.
• Normality Check: Verify the data follows a normal distribution — capability indices assume normality.
• Cp/Cpk Calculation: Calculate short-term (within-subgroup) capability indices.
• Pp/Ppk Calculation: Calculate long-term (overall) performance indices.

Why It Matters

• Prediction: Capability indices predict the expected defect rate — Cpk 1.33 = ~63 PPM, Cpk 2.0 = ~0.002 PPM.
• Automotive: AEC-Q100/IATF 16949 require Cpk ≥ 1.67 for critical parameters — mandatory for automotive qualification.
• Continuous Monitoring: Capability is tracked over time — degrading capability signals process drift before defects appear.

SPC Process Capability is measuring manufacturing precision — quantifying how well the process stays within specifications for predictable, high-quality production.