Statistics Quality Control & Assurance 4 — Questions and Answers
Question 1: In a Double Sampling Plan, what happens if the number of defectives in the first sample falls between the acceptance and rejection numbers?
- The lot is immediately rejected
- The lot is accepted without further inspection
- A second sample is drawn and both samples are evaluated together (Correct answer)
- The entire lot undergoes 100% inspection
Correct answer: A second sample is drawn and both samples are evaluated together
In double sampling, an indeterminate first-sample result triggers a second sample; the combined count from both samples is then compared to final acceptance/rejection criteria.
Question 2: Average Outgoing Quality (AOQ) in acceptance sampling reaches its maximum value, called the AOQL, when:
- The lot fraction defective approaches zero
- The lot fraction defective is at an intermediate level that maximizes the average defective rate after inspection (Correct answer)
- All lots are rejected and 100% inspected
- The sample size equals the lot size
Correct answer: The lot fraction defective is at an intermediate level that maximizes the average defective rate after inspection
AOQ peaks at the AOQL for an intermediate incoming quality level because very good lots are accepted (low defects passed) and very bad lots are rejected and screened (defects removed), but intermediate quality maximizes defects passed.
Question 3: A Pareto chart in quality control is used to:
- Plot defect counts over time to detect trends
- Rank defect types by frequency to identify the vital few causes (Correct answer)
- Display the distribution of a continuous quality characteristic
- Compare the means of two production processes
Correct answer: Rank defect types by frequency to identify the vital few causes
A Pareto chart ranks defect categories in descending frequency, applying the 80/20 rule to focus improvement efforts on the most significant causes.
Question 4: Which control chart should be used when the subgroup size n = 1 (individual measurements)?
- X-bar and R chart
- X-bar and S chart
- Individuals (I) and Moving Range (MR) chart (Correct answer)
- p-chart
Correct answer: Individuals (I) and Moving Range (MR) chart
When only one measurement is taken at each time point, the I-MR chart is used, with the moving range providing an estimate of short-term variation.
Question 5: In the context of Measurement System Analysis, 'reproducibility' refers to:
- The variation when the same operator measures the same part repeatedly
- The variation caused by different operators measuring the same part (Correct answer)
- The agreement between the measured value and the true value
- The ability to detect small differences between parts
Correct answer: The variation caused by different operators measuring the same part
Reproducibility is the variation in measurement averages when different appraisers (operators) measure the same set of parts under the same conditions.
Question 6: A process with Cp = 1.5 and Cpk = 0.8 indicates that the process:
- Is capable and well-centered within specifications
- Has sufficient spread to meet specs but is significantly off-center (Correct answer)
- Is both off-center and has too much variation to meet specs
- Meets the Six Sigma quality standard
Correct answer: Has sufficient spread to meet specs but is significantly off-center
Cp = 1.5 shows the process spread could fit well within the specifications, but Cpk = 0.8 < 1.0 shows the process mean is shifted toward or beyond one specification limit.
Question 7: ISO 9001 is best described as a standard for:
- Specifying product performance requirements for manufactured goods
- Establishing a quality management system framework focused on customer satisfaction and continual improvement (Correct answer)
- Defining statistical methods for process control
- Setting environmental management requirements
Correct answer: Establishing a quality management system framework focused on customer satisfaction and continual improvement
ISO 9001 is a process-based Quality Management System (QMS) standard requiring organizations to demonstrate the ability to consistently provide products/services that meet customer and regulatory requirements.
Statistics Quality Control & Assurance 4
In a Double Sampling Plan, what happens if the number of defectives in the first sample falls between the acceptance and rejection numbers?