Process Capability

Process capability measures how well a process meets specifications. The capability indices Cp and Cpk compare the process spread (6σ) to the specification width. Cp = (USL - LSL) / (6σ) measures potential capability. Cpk = min[(USL - μ)/(3σ), (μ - LSL)/(3σ)] measures actual capability. A Cpk ≥ 1.33 is generally considered capable. Process capability studies are essential for Six Sigma and quality control.

Governing FormulaCp = (USL - LSL) / (6σ), Cpk = min[(USL - μ)/(3σ), (μ - LSL)/(3σ)], Ppk = min[(USL - μ)/(3s), (μ - LSL)/(3s)]

Knowledge Check

10 Questions

1.A process has USL = 110, LSL = 90, mean = 100, and σ = 2. What is Cp?

2.What is the difference between Cp and Cpk?

3.A process has USL = 50, LSL = 30, mean = 42, and σ = 2. What is Cpk?

4.What is the minimum acceptable Cpk value for a capable process?

5.If Cpk = 0.5, what does this indicate?

6.What is the relationship between Cpk and Ppk?

7.A process has Cpk = 1.5. What percentage of defects is expected? (approximately)

8.What is the specification width in terms of σ if Cp = 1.33?

9.Which of the following is NOT needed to calculate Cpk?

10.If Cpk is less than 1.0, the process is said to be: