Lean Six Sigma Black Belt Certification Statistical Process Control (SPC) 3 — Questions and Answers
Question 1: Which control chart is most appropriate for monitoring the number of defects per unit when the subgroup size varies?
- c-chart
- np-chart
- u-chart (Correct answer)
- p-chart
Correct answer: u-chart
The u-chart monitors defects per unit and accommodates variable subgroup sizes, unlike the c-chart which requires a constant area of opportunity.
Question 2: Process capability ratio Cp differs from Cpk in that Cp:
- Accounts for process centering while Cpk does not
- Ignores process centering while Cpk accounts for it (Correct answer)
- Uses within-subgroup sigma while Cpk uses total sigma
- Is calculated only for attribute data
Correct answer: Ignores process centering while Cpk accounts for it
Cp measures only the spread relative to specifications without regard to centering, while Cpk adjusts for how far the process mean is from the nearest specification limit.
Question 3: An EWMA chart uses a smoothing parameter λ (lambda). What happens when λ approaches 1?
- The chart gives more weight to historical data
- The chart behaves more like a Shewhart individuals chart (Correct answer)
- The chart becomes more sensitive to small shifts
- The control limits widen significantly
Correct answer: The chart behaves more like a Shewhart individuals chart
As λ approaches 1, the EWMA chart places nearly all weight on the most recent observation, approximating a Shewhart individuals (I) chart.
Question 4: Which situation would make a P-chart inappropriate?
- When defects per unit are being tracked (Correct answer)
- When subgroup sizes are constant
- When classifying items as conforming or nonconforming
- When the proportion nonconforming is near 0.5
Correct answer: When defects per unit are being tracked
The P-chart monitors the proportion of nonconforming items (pass/fail), not the count of defects per unit; for defects per unit, a u-chart or c-chart should be used.
Question 5: In a Gage R&R study, which component of variation confirms that the measurement system discriminates between parts?
- Repeatability
- Reproducibility
- Part-to-part variation (Correct answer)
- Appraiser-to-appraiser interaction
Correct answer: Part-to-part variation
Part-to-part variation being large relative to measurement system variation confirms the gage can distinguish between different parts, indicating adequate discrimination.
Question 6: Which term describes a process that is in statistical control but produces output outside specification limits?
- Capable and stable
- Stable but not capable (Correct answer)
- Special cause variation
- Out of control
Correct answer: Stable but not capable
A process can be statistically stable (only common cause variation) yet still incapable if the natural process spread exceeds the specification width.
Question 7: What is the effect of using 3-sigma control limits instead of 2-sigma limits on Type I error (false alarm rate)?
- Increases Type I error
- Decreases Type I error (Correct answer)
- Has no effect on Type I error
- Eliminates Type I error entirely
Correct answer: Decreases Type I error
Wider 3-sigma limits reduce the probability of falsely signaling a special cause when only common cause variation is present, thus decreasing the Type I error rate.
Lean Six Sigma Black Belt Certification Statistical Process Control (SPC) 3
Which control chart is most appropriate for monitoring the number of defects per unit when the subgroup size varies?