Process Capability Analysis Flashcards
7 cards from real Certified Six Sigma Black Belt Exam practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
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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.