SPC Fundamentals 2 — Questions and Answers
Question 1: Which of the following best describes 'common cause variation' in a process?
- Variation due to a sudden machine breakdown
- Variation inherent to the process that is always present (Correct answer)
- Variation caused by an untrained operator
- Variation resulting from an incorrect raw material batch
Correct answer: Variation inherent to the process that is always present
Common cause variation is the natural, inherent variability in a process that is always present and results from many small, everyday factors.
Question 2: On an X-bar chart, what does a single point beyond the upper or lower control limit indicate?
- The process is naturally varying
- A potential special cause that should be investigated (Correct answer)
- The control limits need to be recalculated
- The sample size is too small
Correct answer: A potential special cause that should be investigated
A point beyond a control limit is a signal of special cause variation that warrants immediate investigation.
Question 3: What is the primary purpose of a process capability index (Cp)?
- To measure the centering of the process relative to specifications
- To compare the process spread to the specification width (Correct answer)
- To detect special causes of variation
- To estimate the process mean
Correct answer: To compare the process spread to the specification width
Cp compares the specification width (USL − LSL) to the process spread (6σ) to indicate whether the process can fit within specifications.
Question 4: A rational subgroup in SPC is best defined as a group of measurements that:
- Is collected over the longest possible time span
- Minimizes the chance of special causes occurring within the group (Correct answer)
- Contains the maximum allowable number of defects
- Spans multiple process shifts to capture all variation
Correct answer: Minimizes the chance of special causes occurring within the group
Rational subgroups are sampled so that variation within the group reflects only common causes, making between-group variation a reliable signal of process changes.
Question 5: Which control chart is most appropriate for monitoring the number of defects per unit when the subgroup size varies?
- c chart
- np chart
- u chart (Correct answer)
- p chart
Correct answer: u chart
The u chart monitors the average number of defects per unit and accommodates varying subgroup sizes.
Question 6: In SPC, 'control limits' are typically set at:
- The engineering specification limits
- ±1 sigma from the process mean
- ±3 sigma from the process mean (Correct answer)
- ±2 sigma from the process mean
Correct answer: ±3 sigma from the process mean
Control limits are conventionally placed at ±3 sigma from the process mean, providing a 99.73% probability that in-control points fall within them.
Question 7: When a control chart shows 8 consecutive points on the same side of the center line, this is an example of:
- Random variation within control limits
- A run rule signaling a potential process shift (Correct answer)
- Normal sampling error
- A correctly centered process
Correct answer: A run rule signaling a potential process shift
Eight or more consecutive points on one side of the center line is a run rule that signals a potential process mean shift.
SPC Fundamentals 2
Which of the following best describes 'common cause variation' in a process?