Lean Six Sigma Black Belt Certification Statistical Process Control (SPC) 2 — Questions and Answers
Question 1: A process has a Cpk of 0.85. What does this indicate?
- The process is centered and capable
- The process is capable but may not be centered
- The process is not capable of meeting specifications (Correct answer)
- The process has no variation
Correct answer: The process is not capable of meeting specifications
A Cpk below 1.0 indicates the process is not capable of consistently meeting specification limits.
Question 2: Which Western Electric rule is violated when 2 out of 3 consecutive points fall beyond 2 sigma on the same side of the centerline?
- Rule 1
- Rule 2 (Correct answer)
- Rule 3
- Rule 4
Correct answer: Rule 2
Western Electric Rule 2 flags 2 out of 3 consecutive points beyond 2 sigma on the same side as a signal of special cause.
Question 3: What is the primary purpose of the CUSUM chart?
- Detect large, sudden shifts in the mean
- Detect small, sustained shifts in the mean over time (Correct answer)
- Monitor process variability
- Plot attribute data proportions
Correct answer: Detect small, sustained shifts in the mean over time
CUSUM (Cumulative Sum) charts are specifically designed to detect small, sustained process mean shifts that Shewhart charts may miss.
Question 4: In an np-chart, what does 'n' represent?
- Number of defects per unit
- Number of nonconforming items in a subgroup
- Constant subgroup size (Correct answer)
- Number of opportunities for defects
Correct answer: Constant subgroup size
In an np-chart, 'n' is the constant subgroup size; the chart plots the count of nonconforming items using a fixed sample size.
Question 5: Which measure describes the within-subgroup short-term variation used to estimate sigma in control chart calculations?
- Standard deviation of all individual values
- Average range (R-bar) (Correct answer)
- Total process standard deviation
- Median of all measurements
Correct answer: Average range (R-bar)
The average range (R-bar) captures within-subgroup variation and is used to estimate the short-term process standard deviation for control limit calculations.
Question 6: A team notices a gradual upward trend of 7 consecutive points on an X-bar chart. Which type of special cause is most likely?
- Outlier from a measurement error
- Tool wear or process drift (Correct answer)
- Sudden input material change
- Operator-to-operator variation
Correct answer: Tool wear or process drift
A sustained upward trend on a control chart typically indicates process drift, often caused by tool wear, gradual equipment degradation, or consumable depletion.
Question 7: What does the d2 constant used in control chart formulas represent?
- The ratio of process spread to specification width
- A factor that converts the average range to an estimate of sigma (Correct answer)
- The number of standard deviations between control limits
- The correction factor for small sample sizes
Correct answer: A factor that converts the average range to an estimate of sigma
The d2 constant is a bias correction factor that converts R-bar to an unbiased estimate of the process standard deviation (sigma-hat = R-bar / d2).
Lean Six Sigma Black Belt Certification Statistical Process Control (SPC) 2
A process has a Cpk of 0.85.
What does this indicate?