CPP Statistical Process Control 2 — Questions and Answers
Question 1: Which type of control chart is best suited 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 tracks defects per unit and accommodates variable sample sizes, making it appropriate when inspection units differ in size.
Question 2: A process is said to be in a state of statistical control when:
- All output meets customer specifications
- Only common cause variation is present (Correct answer)
- The Cpk value exceeds 1.33
- No defects are produced in the last 30 samples
Correct answer: Only common cause variation is present
Statistical control means the process exhibits only common cause (random) variation, with no special causes influencing the output.
Question 3: On an X-bar chart, what does the Western Electric Rule 2 (two of three points beyond 2 sigma) signal?
- A random pattern with no concern
- A potential shift in process mean requiring investigation (Correct answer)
- The control limits need to be recalculated
- The process capability is adequate
Correct answer: A potential shift in process mean requiring investigation
Two of three consecutive points beyond 2-sigma on the same side signals a probable shift in the process mean and should trigger investigation.
Question 4: What is the primary purpose of the R-chart used alongside an X-bar chart?
- Track the process average over time
- Monitor within-subgroup variation (spread) (Correct answer)
- Detect attribute defects in the process
- Calculate the process capability index
Correct answer: Monitor within-subgroup variation (spread)
The R-chart monitors variability within subgroups, ensuring the spread of the process remains stable alongside tracking the mean.
Question 5: A manufacturer samples 50 units each shift and records defective units. Which control chart is most appropriate?
- u-chart
- c-chart
- np-chart (Correct answer)
- X-bar & S chart
Correct answer: np-chart
The np-chart is used for count of defective items with a constant sample size, making it ideal for this fixed-subgroup scenario.
Question 6: In SPC, a 'run' of eight consecutive points on one side of the centerline most likely indicates:
- Acceptable common cause variation
- A special cause that has shifted the process (Correct answer)
- The process is highly capable
- Control limits are set too wide
Correct answer: A special cause that has shifted the process
Eight or more consecutive points on one side of the centerline is a run rule indicating a non-random pattern and probable special cause.
Question 7: What does the D4 constant represent in control chart calculations for an X-bar and R chart?
- Lower control limit factor for the R-chart
- Upper control limit factor for the R-chart (Correct answer)
- Center line factor for the X-bar chart
- Process capability multiplier
Correct answer: Upper control limit factor for the R-chart
D4 is the factor multiplied by the average range (R-bar) to compute the upper control limit on the R-chart.
CPP Statistical Process Control 2
Which type of control chart is best suited for monitoring the number of defects per unit when the sample size varies?