CRE CRE Corrective Action and Continuous Improvement 2 — Questions and Answers
Question 1: The primary Six Sigma metric for measuring process capability is expressed as:
- Mean Time Between Failures (MTBF)
- Process sigma level, where 6σ corresponds to 3.4 defects per million opportunities (Correct answer)
- Reliability coefficient R(t) at mission time
- FMEA Risk Priority Number (RPN)
Correct answer: Process sigma level, where 6σ corresponds to 3.4 defects per million opportunities
Six Sigma defines process quality in terms of sigma level; a 6σ process produces only 3.4 DPMO.
Question 2: On a standard control chart, the Upper Control Limit (UCL) is typically set at:
- 1 standard deviation above the process mean
- 3 standard deviations above the process mean (Correct answer)
- The upper engineering specification limit
- 2 standard deviations above the process mean
Correct answer: 3 standard deviations above the process mean
Control limits at ±3σ from the mean represent the bounds of natural process variation with approximately 99.73% of data within limits.
Question 3: Which type of control chart is used to monitor the fraction (proportion) of defective items in a process?
- X-bar (mean) chart
- R (range) chart
- p-chart (Correct answer)
- c-chart
Correct answer: p-chart
The p-chart monitors the proportion of nonconforming items in each sample and is used for attribute data.
Question 4: Design of Experiments (DOE) in reliability improvement is used to:
- Create formal design documentation packages
- Systematically study the effect of multiple factors on a reliability response (Correct answer)
- Calculate field MTBF from warranty return data
- Generate test plans for environmental stress screening
Correct answer: Systematically study the effect of multiple factors on a reliability response
DOE allows engineers to efficiently evaluate main effects and interactions of multiple variables on reliability simultaneously.
Question 5: Reliability Growth testing using the Duane postulate tracks improvement by monitoring:
- Reduction in production and test costs over time
- Increasing cumulative MTBF as failures are discovered and corrected during development (Correct answer)
- Total number of test hours consumed
- Reduction in component count through design simplification
Correct answer: Increasing cumulative MTBF as failures are discovered and corrected during development
The Duane model plots cumulative failures versus cumulative test time on log-log paper to demonstrate reliability improvement.
Question 6: A lessons-learned database in a reliability program primarily serves to:
- Satisfy ISO 9001 record retention requirements
- Capture knowledge from past failures and successes to prevent recurrence on future programs (Correct answer)
- Track customer warranty claims and associated costs
- Monitor supplier delivery and quality performance
Correct answer: Capture knowledge from past failures and successes to prevent recurrence on future programs
Lessons-learned systems institutionalize engineering knowledge so that previously solved problems are not repeated on new designs.
CRE CRE Corrective Action and Continuous Improvement 2
The primary Six Sigma metric for measuring process capability is expressed as: