CQI CQI Statistical Process Control 2 — Questions and Answers
Question 1: What does process capability index Cp measure?
- The centering of the process relative to specifications
- The spread of the process relative to the specification width, without regard to centering (Correct answer)
- The number of defects per million opportunities
- The stability of the process over time
Correct answer: The spread of the process relative to the specification width, without regard to centering
Cp measures the ratio of the specification width to the process spread (6σ), indicating potential capability without considering how well the process is centered.
Question 2: What is the minimum acceptable Cpk value for most manufacturing processes?
- 0.5
- 1.0
- 1.33 (Correct answer)
- 2.0
Correct answer: 1.33
A Cpk of 1.33 is the generally accepted minimum for a capable process, providing approximately 99.994% conformance and is required by many automotive standards.
Question 3: What does a Cpk value of 1.0 indicate?
- The process is highly capable
- The nearest specification limit is exactly 3 standard deviations from the process mean (Correct answer)
- The process produces zero defects
- The process is out of statistical control
Correct answer: The nearest specification limit is exactly 3 standard deviations from the process mean
A Cpk of 1.0 means the process mean is exactly 3 standard deviations from the nearest specification limit, yielding approximately 0.27% nonconformance.
Question 4: What is the key difference between Cp and Cpk?
- Cp uses control limits; Cpk uses specification limits
- Cp ignores centering while Cpk accounts for how well the process is centered within specifications (Correct answer)
- Cp is for attribute data; Cpk is for variable data
- There is no practical difference between them
Correct answer: Cp ignores centering while Cpk accounts for how well the process is centered within specifications
Cp measures potential capability (spread only), while Cpk measures actual capability by accounting for both the process spread and its centering within the specification.
Question 5: Process capability studies should only be conducted when:
- The process is in a state of statistical control (Correct answer)
- Special causes of variation are present
- The process is being shut down for maintenance
- Equipment is undergoing calibration
Correct answer: The process is in a state of statistical control
Capability indices are only meaningful when the process is stable (in statistical control), because special cause variation invalidates capability calculations.
Question 6: What does DPMO stand for in quality measurement?
- Defective Parts Monitored per Operation
- Defects Per Million Opportunities (Correct answer)
- Daily Process Monitoring Output
- Dimensional Process Measurement Overview
Correct answer: Defects Per Million Opportunities
DPMO (Defects Per Million Opportunities) is a quality metric used in Six Sigma that normalizes defect rates by accounting for the complexity of a product or process.
CQI CQI Statistical Process Control 2
What does process capability index Cp measure?