Lean Six Sigma Black Belt Certification FREE Lean Six Sigma Black Belt Statistical Process Control (SPC) Questions and Answers 2 — Questions and Answers
Question 1: What is the primary purpose of a CUSUM chart in SPC?
- To detect small, sustained shifts in the process mean over time (Correct answer)
- To monitor the range of subgroup measurements
- To identify seasonal patterns in production data
- To replace traditional X-bar and R charts entirely
Correct answer: To detect small, sustained shifts in the process mean over time
CUSUM (Cumulative Sum) charts are specifically designed to detect small, sustained shifts in the process mean that might go unnoticed on standard Shewhart control charts.
Question 2: In an X-bar and S chart, when is the S chart preferred over the R chart?
- When subgroup sizes are greater than 8 or 9 (Correct answer)
- When the process is highly capable
- When only attribute data is available
- When the sample size is exactly 2
Correct answer: When subgroup sizes are greater than 8 or 9
The S chart is preferred over the R chart when subgroup sizes exceed 8 or 9 because the range becomes a less efficient estimator of process variability with larger subgroups.
Question 3: A process has a Cpk of 1.33 and a Cp of 1.33. What can be concluded?
- The process is both capable and centered within the specification limits (Correct answer)
- The process is capable but shifted off-center
- The process is not capable regardless of centering
- More data is needed to draw any conclusion
Correct answer: The process is both capable and centered within the specification limits
When Cpk equals Cp, the process mean is perfectly centered between the specification limits, and a value of 1.33 indicates the process is capable with a 4-sigma spread relative to specifications.
Question 4: Which Western Electric rule detects a trend pattern on a control chart?
- Six consecutive points steadily increasing or decreasing (Correct answer)
- Two out of three consecutive points beyond 2 sigma on the same side
- One point beyond 3 sigma from the centerline
- Eight consecutive points on one side of the centerline
Correct answer: Six consecutive points steadily increasing or decreasing
The Western Electric rule for trends identifies six or more consecutive points that are steadily increasing or steadily decreasing, indicating a systematic process shift.
Question 5: What does an np chart monitor that a p chart does not require?
- A constant sample size across all subgroups (Correct answer)
- Variable sample sizes across subgroups
- Continuous measurement data
- Multiple defects per unit
Correct answer: A constant sample size across all subgroups
An np chart plots the actual number of defective items and requires a constant sample size, whereas a p chart plots the proportion defective and can handle variable sample sizes.
Question 6: When rational subgrouping is applied in SPC, what is the key objective?
- To maximize variation between subgroups and minimize variation within subgroups (Correct answer)
- To maximize variation within subgroups and minimize variation between subgroups
- To ensure all subgroups have equal means
- To eliminate all common cause variation from the process
Correct answer: To maximize variation between subgroups and minimize variation within subgroups
Rational subgrouping aims to capture only common cause variation within each subgroup while allowing assignable cause variation to appear as differences between subgroups.
Lean Six Sigma Black Belt Certification FREE Lean Six Sigma Black Belt Statistical Process Control (SPC) Questions and Answers 2
What is the primary purpose of a CUSUM chart in SPC?