Quality Auditor Certification Statistical Sampling 4 — Questions and Answers
Question 1: The Average Outgoing Quality (AOQ) curve reaches a maximum value called AOQL. What does the AOQL represent?
- The best possible outgoing quality achievable with any plan
- The worst average outgoing quality that can result from a rectifying inspection plan (Correct answer)
- The incoming quality level that triggers tightened inspection
- The defect rate at which 50% of lots are rejected
Correct answer: The worst average outgoing quality that can result from a rectifying inspection plan
The AOQL is the peak of the AOQ curve, representing the maximum long-run average fraction defective in outgoing product.
Question 2: In cluster sampling, the sampling unit is:
- An individual item chosen from the full population
- A naturally occurring group, with all or a random subset of items within selected groups inspected (Correct answer)
- A stratum of homogeneous items
- Every kth item in a sequence
Correct answer: A naturally occurring group, with all or a random subset of items within selected groups inspected
Cluster sampling selects groups (clusters) randomly and then inspects all or a sample of items within each selected cluster.
Question 3: A quality auditor finds that a supplier consistently submits lots near the AQL. Under ANSI/ASQ Z1.4, which inspection level is appropriate once a good track record is established?
- Tightened inspection
- Reduced inspection (Correct answer)
- Normal inspection
- Discontinue inspection
Correct answer: Reduced inspection
Reduced inspection is triggered after a specified number of consecutive lots pass under normal inspection, rewarding consistent quality.
Question 4: The central limit theorem is important in quality sampling because it states that:
- All quality characteristics follow a normal distribution
- The distribution of sample means approaches normality as sample size increases, regardless of population shape (Correct answer)
- Defect counts always follow a Poisson distribution
- Standard deviation decreases to zero as n increases
Correct answer: The distribution of sample means approaches normality as sample size increases, regardless of population shape
The CLT guarantees that sample means are approximately normally distributed for large n, enabling use of normal-based inference.
Question 5: Producer's risk (α) in acceptance sampling is typically set at 5%. This means:
- 5% of all defective lots will be accepted
- There is a 5% chance of rejecting a lot at the AQL quality level (Correct answer)
- 5% of the population is assumed defective
- The sample size covers 5% of the lot
Correct answer: There is a 5% chance of rejecting a lot at the AQL quality level
Alpha (α) is the probability that a lot meeting the AQL is incorrectly rejected, penalizing the producer.
Question 6: Which sampling plan type generally requires the smallest total sample size to achieve equivalent protection compared to single and double sampling?
- Single sampling plan
- Double sampling plan
- Multiple (sequential) sampling plan (Correct answer)
- Skip-lot sampling plan
Correct answer: Multiple (sequential) sampling plan
Multiple or sequential sampling plans typically achieve the same OC curve protection with fewer total units inspected on average.
Question 7: When auditing a warehouse with 10 storage aisles each containing 500 items, selecting 3 aisles at random and inspecting all items in those aisles is an example of:
- Stratified random sampling
- Systematic sampling
- Simple random sampling
- One-stage cluster sampling (Correct answer)
Correct answer: One-stage cluster sampling
Selecting whole clusters (aisles) at random and inspecting all items within them is one-stage cluster sampling.
Quality Auditor Certification Statistical Sampling 4
The Average Outgoing Quality (AOQ) curve reaches a maximum value called AOQL.
What does the AOQL represent?