SPC Control Charts & Analysis 1 — Questions and Answers
Question 1: What is the purpose of the center line on a control chart?
- Upper specification limit
- Process target
- Process mean (Correct answer)
- Median of all samples
Correct answer: Process mean
The center line represents the average or mean of the process and serves as a reference for detecting variation.
Question 2: What does it indicate when points trend steadily upward on a control chart?
- Improved efficiency
- Stable process
- Possible process shift (Correct answer)
- Random variation
Correct answer: Possible process shift
A continuous upward trend may signal a shift in the process mean, requiring investigation.
Question 3: Which type of control chart is best for monitoring process variability?
- P-chart
- X-bar chart
- R-chart (Correct answer)
- Histogram
Correct answer: R-chart
R-charts monitor the range of variation within subgroups, making them useful for detecting shifts in variability.
Question 4: How many standard deviations from the mean are typical control limits set at?
- 1
- 2
- 3 (Correct answer)
- 4
Correct answer: 3
Control limits are generally placed at ±3 standard deviations to detect statistically significant variation.
Question 5: Which of the following may indicate a process is out of control?
- All points near the center line
- Even distribution of points
- One point outside control limits (Correct answer)
- Stable control limits
Correct answer: One point outside control limits
A single point outside the control limits is a common signal that the process may be out of control.
Question 6: What is the main difference between control limits and specification limits?
- Control limits are wider
- Control limits are process-based (Correct answer)
- Specification limits are based on samples
- Specification limits change over time
Correct answer: Control limits are process-based
Control limits reflect process performance; specification limits are customer or design requirements.
SPC Control Charts & Analysis 1
What is the purpose of the center line on a control chart?