Quality Auditor Certification Statistical Sampling 2 — Questions and Answers
Question 1: A quality auditor wants to estimate the proportion of defective items in a large lot with 95% confidence and ±3% precision. Which factor directly determines the required sample size?
- The lot size only
- The estimated proportion and desired margin of error (Correct answer)
- The inspection speed of the auditor
- The cost per unit inspected
Correct answer: The estimated proportion and desired margin of error
Required sample size is determined by the estimated defect proportion, desired confidence level, and acceptable margin of error.
Question 2: In skip-lot sampling, what condition must be met before a supplier qualifies for reduced inspection frequency?
- The supplier must be ISO 9001 certified
- A specified number of consecutive lots must pass inspection (Correct answer)
- The lot size must exceed 5,000 units
- The customer must waive all quality requirements
Correct answer: A specified number of consecutive lots must pass inspection
Skip-lot sampling requires a demonstrated history of acceptable consecutive lots before reducing inspection frequency.
Question 3: Which sampling method ensures every kth unit is selected after a random start, making it efficient for production-line inspection?
- Cluster sampling
- Stratified sampling
- Systematic sampling (Correct answer)
- Judgment sampling
Correct answer: Systematic sampling
Systematic sampling selects every kth item after a random starting point, making it practical for continuous production.
Question 4: The operating characteristic (OC) curve for a sampling plan shows the relationship between:
- Sample size and inspection cost
- Lot defect rate and probability of acceptance (Correct answer)
- Producer's risk and consumer's risk only
- AQL and LTPD on the same axis
Correct answer: Lot defect rate and probability of acceptance
The OC curve plots the probability of accepting a lot as a function of the true lot fraction defective.
Question 5: A process produces 2% defective parts. If a sample of 100 units is drawn and the acceptance number is 5, what describes the probability of rejecting a good lot?
- Consumer's risk (beta)
- Producer's risk (alpha) (Correct answer)
- Average outgoing quality limit
- Lot tolerance percent defective
Correct answer: Producer's risk (alpha)
Producer's risk (alpha) is the probability of rejecting a lot that meets the quality standard (AQL).
Question 6: When auditing records, a quality auditor uses stratified random sampling by department. What is the primary advantage of this approach?
- It eliminates the need for a sampling frame
- It reduces variability within each stratum and improves precision (Correct answer)
- It requires the smallest possible sample size
- It avoids any form of selection bias entirely
Correct answer: It reduces variability within each stratum and improves precision
Stratified sampling improves estimate precision by ensuring each subgroup is adequately represented and reducing within-group variability.
Question 7: Which statistic measures the spread of the sampling distribution of the mean, and decreases as sample size increases?
- Population standard deviation
- Standard error of the mean (Correct answer)
- Coefficient of variation
- Range of the sample
Correct answer: Standard error of the mean
The standard error of the mean equals the population standard deviation divided by the square root of n, shrinking as n grows.
Quality Auditor Certification Statistical Sampling 2
A quality auditor wants to estimate the proportion of defective items in a large lot with 95% confidence and ±3% precision.
Which factor directly determines the required sample size?