CQE Statistical Process Control 1 — Questions and Answers
Question 1: What does a control chart's Upper Control Limit (UCL) represent in Statistical Process Control?
- The specification limit set by the customer
- Three standard deviations above the process mean (Correct answer)
- The maximum allowable defect rate
- The target value for the process
Correct answer: Three standard deviations above the process mean
The UCL is calculated as three standard deviations above the process mean, representing the upper boundary of natural process variation.
Question 2: Which control chart is most appropriate for monitoring the number of defects per unit when the sample size varies?
- p-chart
- c-chart
- u-chart (Correct answer)
- np-chart
Correct answer: u-chart
The u-chart monitors the average number of defects per unit and accommodates variable sample sizes, making it ideal when inspection units differ in size.
Question 3: A process capability index (Cpk) of 1.33 indicates the process is:
- Barely capable of meeting specifications
- Capable with a safety margin (Correct answer)
- Out of control
- Centered exactly on the target
Correct answer: Capable with a safety margin
A Cpk of 1.33 corresponds to a 4-sigma process, meaning the process is capable with a reasonable safety margin beyond the minimum threshold of 1.0.
Question 4: What is the primary purpose of a Western Electric (WECO) rule in SPC?
- To set specification limits
- To detect non-random patterns indicating a special cause (Correct answer)
- To calculate the process sigma level
- To determine sample size
Correct answer: To detect non-random patterns indicating a special cause
WECO rules are supplementary run rules applied to control charts to detect non-random patterns that signal special cause variation beyond just points outside control limits.
Question 5: In an X-bar and R chart, which component measures within-subgroup variation?
- X-bar chart
- R chart (Correct answer)
- Both charts equally
- Neither chart directly
Correct answer: R chart
The R (Range) chart tracks the within-subgroup variation by plotting the range of each subgroup, while the X-bar chart tracks between-subgroup variation.
Question 6: A process with Cp = 1.5 but Cpk = 0.8 tells you that the process is:
- Capable and well-centered
- Highly capable but significantly off-center (Correct answer)
- Incapable regardless of centering
- Correctly centered but has high variation
Correct answer: Highly capable but significantly off-center
A high Cp with a low Cpk indicates the process spread is small enough to meet specs but the process mean is shifted significantly off target.
CQE Statistical Process Control 1
What does a control chart's Upper Control Limit (UCL) represent in Statistical Process Control?