CQI CQI Statistical Process Control 1 — Questions and Answers
Question 1: What is the primary purpose of a control chart?
- To identify the root cause of defects
- To distinguish between common cause and special cause variation (Correct answer)
- To set product specifications
- To calculate process yield
Correct answer: To distinguish between common cause and special cause variation
Control charts are used to monitor processes over time and distinguish between normal (common cause) variation and abnormal (special cause) variation.
Question 2: Which control chart is used to monitor the proportion of defective items in a sample?
- X-bar chart
- R chart
- p-chart (Correct answer)
- c-chart
Correct answer: p-chart
The p-chart monitors the proportion of nonconforming units in samples and is an attribute control chart for fraction defective.
Question 3: What do the Upper Control Limit (UCL) and Lower Control Limit (LCL) represent on a control chart?
- The product specification limits
- The boundaries within which process variation is expected due to common causes (±3σ) (Correct answer)
- The customer's acceptable quality level
- The maximum and minimum allowable measurements
Correct answer: The boundaries within which process variation is expected due to common causes (±3σ)
UCL and LCL are calculated at ±3 standard deviations from the process mean and define the expected range of common cause variation.
Question 4: Which of the following is a signal of special cause variation on a control chart?
- Points randomly scattered within control limits
- Eight consecutive points on the same side of the centerline (Correct answer)
- Equal numbers of points above and below the centerline
- Points evenly distributed across the chart
Correct answer: Eight consecutive points on the same side of the centerline
Eight consecutive points on the same side of the centerline (a Nelson/Western Electric rule) indicate a process mean shift, signaling special cause variation.
Question 5: An X-bar and R chart monitors:
- Attribute data (pass/fail)
- Variable (continuous) data — subgroup means and ranges (Correct answer)
- Count data (number of defects)
- Proportion defective data
Correct answer: Variable (continuous) data — subgroup means and ranges
The X-bar and R chart monitors variable (continuous) data by tracking subgroup averages (X-bar) and ranges (R) to detect process shifts and spread changes.
Question 6: What is the typical subgroup size used with an X-bar and R chart?
- 1
- 2 to 5 (Correct answer)
- 50 to 100
- 500 or more
Correct answer: 2 to 5
X-bar and R charts are typically used with subgroup sizes of 2 to 5 measurements, which is sufficient to estimate variation and detect process shifts.
CQI CQI Statistical Process Control 1
What is the primary purpose of a control chart?