Lean Six Sigma Black Belt Certification Control Phase: SPC 3 — Questions and Answers
Question 1: Which of the following best describes the CUSUM chart's advantage over a traditional Shewhart X-bar chart?
- It uses wider control limits to reduce false alarms
- It is more sensitive to small, sustained shifts in the process mean (Correct answer)
- It does not require normally distributed data
- It monitors both mean and variance simultaneously
Correct answer: It is more sensitive to small, sustained shifts in the process mean
CUSUM (Cumulative Sum) charts accumulate deviations from the target and are highly sensitive to small sustained shifts that Shewhart charts often miss.
Question 2: The d2 constant in control chart formulas is used to:
- Estimate the process standard deviation from the average range (Correct answer)
- Calculate the center line of a p chart
- Convert Cp to Cpk
- Determine rational subgroup size
Correct answer: Estimate the process standard deviation from the average range
d2 is a control chart constant that converts the average range (R-bar) to an estimate of the process standard deviation: σ̂ = R-bar / d2.
Question 3: A short-run SPC technique is used when:
- The process produces thousands of identical parts per run
- Production runs are too short to establish traditional control limits (Correct answer)
- The measurement system has high gauge R&R error
- Control limits are unknown and must be set from historical data
Correct answer: Production runs are too short to establish traditional control limits
Short-run SPC methods (e.g., standardized charts, DNOM charts) are designed for job-shop environments where each run produces too few parts for standard control limits.
Question 4: On an EWMA chart, a smaller smoothing parameter λ (lambda) results in:
- Greater sensitivity to large sudden shifts
- Faster response to individual extreme data points
- Higher sensitivity to small, gradual process shifts (Correct answer)
- Wider control limits
Correct answer: Higher sensitivity to small, gradual process shifts
A smaller λ gives more weight to historical data, making the EWMA chart more sensitive to small, sustained shifts and less reactive to individual outliers.
Question 5: When a process is described as 'in statistical control,' it means:
- All output meets customer specification limits
- Only common cause variation is present (Correct answer)
- Cp and Cpk both exceed 1.33
- No defective parts are being produced
Correct answer: Only common cause variation is present
Statistical control means the process exhibits only common cause (random) variation with no special causes present, not necessarily that all parts are within spec.
Question 6: Which SPC chart would be selected to monitor the proportion of defective items when subgroup size is constant at n=50?
- c chart
- u chart
- np chart (Correct answer)
- X-bar and S chart
Correct answer: np chart
The np chart monitors the number (count) of defectives in a fixed-size subgroup, making it appropriate when n is constant.
Question 7: A control chart for individual measurements (I-MR chart) uses a moving range of 2 consecutive values. The control limits for the individuals chart use which constant?
- A2
- E2 (Correct answer)
- D4
- c4
Correct answer: E2
The I chart uses the E2 constant (E2 = 2.66 for n=2 moving range) to calculate control limits: UCL/LCL = X-double-bar ± E2 × MR-bar.
Lean Six Sigma Black Belt Certification Control Phase: SPC 3
Which of the following best describes the CUSUM chart's advantage over a traditional Shewhart X-bar chart?