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.