CQE Statistical Process Control 2 — Questions and Answers
Question 1: Which type of variation is inherent to a stable process and cannot be eliminated without changing the process itself?
- Special cause variation
- Assignable cause variation
- Common cause variation (Correct answer)
- Systematic variation
Correct answer: Common cause variation
Common cause variation is the natural, random variation built into the process design, and reducing it requires fundamental process changes rather than firefighting.
Question 2: What does an OC (Operating Characteristic) curve display in acceptance sampling?
- The relationship between sample size and cost
- The probability of accepting a lot at various quality levels (Correct answer)
- The control limits for a process
- The defect rate over time
Correct answer: The probability of accepting a lot at various quality levels
An OC curve plots the probability of lot acceptance against the actual incoming lot quality (fraction defective), helping evaluate sampling plan performance.
Question 3: In SPC, a 'run of 8 consecutive points on one side of the centerline' is an example of:
- Common cause variation
- A natural random pattern
- A signal of special cause variation (Correct answer)
- An acceptable process performance
Correct answer: A signal of special cause variation
Eight consecutive points on one side of the centerline is a classic run rule that signals a process shift or special cause, as the probability of this occurring by chance is very low.
Question 4: Short-term process capability is typically measured using data collected over what period?
- Several months of production
- A single brief study period under stable conditions (Correct answer)
- Annual production totals
- Multiple shifts across different operators
Correct answer: A single brief study period under stable conditions
Short-term capability (Cp/Cpk) is measured during a controlled study with minimal time-to-time variation, representing the best the process can achieve.
Question 5: What statistical distribution is typically assumed for attribute control charts such as the p-chart?
- Normal distribution
- Poisson distribution
- Binomial distribution (Correct answer)
- Exponential distribution
Correct answer: Binomial distribution
The p-chart is based on the binomial distribution, which models the probability of a certain number of nonconforming items in a sample when each item is independently classified as pass or fail.
Question 6: Which index would you use if a process has a one-sided specification limit only?
- Cp
- Cpk
- Cpm
- CPU or CPL individually (Correct answer)
Correct answer: CPU or CPL individually
When only one specification limit exists, you use either CPU (upper) or CPL (lower) alone, since Cp requires both limits and Cpk takes the minimum of both.
CQE Statistical Process Control 2
Which type of variation is inherent to a stable process and cannot be eliminated without changing the process itself?