Which statement best describes the relationship between Cp and Cpk in process capability analysis?

Which statement best describes the relationship between Cp and Cpk in process capability analysis?

• A. Cp measures potential capability only, while Cpk measures market performance.
• B. Cp measures potential process capability; Cpk measures actual capability considering centering; values ≥1.33 indicate acceptable capability.
• C. Cp measures actual performance of the process; Cpk measures process stability; values ≥2.0 indicate capability.
• D. Cp and Cpk are the same metric and always equal.

Answer: (B)

Cp shows what the process could achieve in theory by looking at the spread of the data relative to the specification width, assuming the process is centered and stable. It reflects potential capability, not whether the process is actually centered. Cpk, on the other hand, measures actual capability by taking into account where the process mean sits relative to the specification limits and the process spread; it’s effectively the observed capability when centering is considered. If the process is perfectly centered, Cp and Cpk are the same; if the mean shifts toward a limit, Cpk drops below Cp because the closer side reduces how well the process fits within specs. A common benchmark is that values of at least 1.33 indicate acceptable capability, though the exact requirement depends on the application.

Cp shows what the process could achieve in theory by looking at the spread of the data relative to the specification width, assuming the process is centered and stable. It reflects potential capability, not whether the process is actually centered. Cpk, on the other hand, measures actual capability by taking into account where the process mean sits relative to the specification limits and the process spread; it’s effectively the observed capability when centering is considered. If the process is perfectly centered, Cp and Cpk are the same; if the mean shifts toward a limit, Cpk drops below Cp because the closer side reduces how well the process fits within specs. A common benchmark is that values of at least 1.33 indicate acceptable capability, though the exact requirement depends on the application.