Process Capability Analysis Flashcards

What does the Cp index measure in process capability analysis?

Answer: Process spread relative to specification width

Cp = (USL - LSL) / (6σ) measures the ratio of the specification width to the process spread, indicating potential capability regardless of centering.

A process has Cp = 1.5 and Cpk = 0.8. What does this indicate?

Answer: The process has sufficient spread but is significantly off-center

Cp = 1.5 indicates sufficient process spread potential, but Cpk = 0.8 (below 1.0) reveals the process mean is significantly shifted away from the specification center.

What is the standard minimum acceptable Cpk value for a process to be considered 'capable' in most Six Sigma guidelines?

Answer: 1.33

A Cpk of 1.33 corresponds to a 4-sigma process producing approximately 64 DPMO and is the widely accepted minimum threshold for process capability.

Which capability index accounts for BOTH process spread AND centering simultaneously?

Answer: Cpk

Cpk = min[(USL − μ)/3σ, (μ − LSL)/3σ] takes the minimum of both one-sided indices, penalizing any shift of the mean toward either specification limit.

What is the key difference between Cpk and Ppk?

Answer: Cpk uses within-subgroup (short-term) variation; Ppk uses total (long-term) variation

Cpk uses within-subgroup standard deviation (short-term, potential capability), while Ppk uses overall standard deviation from all data (long-term, actual performance).

When Cp equals Cpk for a process, what does this tell you about the process?

Answer: The process mean is exactly centered between the specification limits

Cp equals Cpk only when the process mean is exactly at the midpoint of the specification limits, making both one-sided distances equal.

Using the relationship Z = 3 × Cpk, what sigma level corresponds to a Cpk of 1.33?

Answer: 4 sigma

Applying Z = 3 × Cpk gives Z = 3 × 1.33 = 4, meaning the process operates at the 4 sigma level.