CPP Statistical Process Control 3 — Questions and Answers
Question 1: Process capability index Cp measures:
- How centered the process is within specification limits
- The ratio of specification width to process spread (Correct answer)
- The percentage of defective output
- The number of standard deviations to the nearest spec limit
Correct answer: The ratio of specification width to process spread
Cp compares the total specification width (USL − LSL) to the process spread (6 sigma), indicating whether the process fits within tolerances.
Question 2: A process has a Cp of 1.5 but a Cpk of 0.8. What does this indicate?
- The process is both capable and centered
- The process spread fits specs but it is not centered (Correct answer)
- The process is incapable and needs wider limits
- Cpk and Cp values are contradictory and invalid
Correct answer: The process spread fits specs but it is not centered
A high Cp with a low Cpk means the process spread is narrow enough but the process mean is off-center, pushing output toward one specification limit.
Question 3: Which SPC chart would you use to monitor the time between failures in a low-defect manufacturing environment?
- p-chart
- g-chart (Correct answer)
- c-chart
- X-bar & R chart
Correct answer: g-chart
The g-chart (geometric chart) monitors the number of events or time between rare occurrences, suitable for very low defect rate processes.
Question 4: Common cause variation in a process is best addressed by:
- Identifying and eliminating specific root causes immediately
- Adjusting the process only when a point exceeds control limits
- Fundamentally redesigning or improving the process system (Correct answer)
- Increasing the inspection frequency
Correct answer: Fundamentally redesigning or improving the process system
Common causes are inherent to the system and can only be reduced through systematic process improvement or redesign, not by reacting to individual points.
Question 5: What is the effect of increasing subgroup size (n) on the control limits of an X-bar chart?
- Control limits widen, making the chart less sensitive
- Control limits narrow, making the chart more sensitive (Correct answer)
- Control limits remain unchanged regardless of n
- Only the centerline shifts; limits stay the same
Correct answer: Control limits narrow, making the chart more sensitive
Larger subgroup sizes reduce the standard error of the mean, narrowing control limits and making the chart more sensitive to process shifts.
Question 6: In Phase I of control chart implementation, what is the primary objective?
- Monitoring the process in real time to detect shifts
- Establishing stable baseline control limits from historical data (Correct answer)
- Calculating the process capability index
- Communicating SPC results to customers
Correct answer: Establishing stable baseline control limits from historical data
Phase I uses historical process data to identify and remove special causes, establishing stable control limits that represent the process baseline.
Question 7: A Six Sigma process has approximately how many defects per million opportunities (DPMO)?
- 3.4 (Correct answer)
- 66,807
- 233
- 1,350
Correct answer: 3.4
A Six Sigma process, accounting for a 1.5-sigma mean shift, produces approximately 3.4 defects per million opportunities.
CPP Statistical Process Control 3
Process capability index Cp measures: