SPC Process Variation & Capability 2 — Questions and Answers
Question 1: A process has a Cpk of 0.85. What does this indicate about the process?
- The process is centered and highly capable
- The process is capable but not centered within specifications
- The process is not capable and may be producing defects (Correct answer)
- The process has no measurable variation
Correct answer: The process is not capable and may be producing defects
A Cpk below 1.0 indicates the process is not capable and is likely producing output outside specification limits.
Question 2: Which type of variation is represented by a single point falling outside the control limits on an X-bar chart?
- Common cause variation
- Special cause variation (Correct answer)
- Inherent variation
- Natural variation
Correct answer: Special cause variation
Points outside control limits are a signal of special cause (assignable cause) variation that requires investigation.
Question 3: The Pp index differs from Cp in that Pp uses:
- The estimated sigma based on control chart subgroups
- The overall standard deviation of all collected data (Correct answer)
- The median range to estimate spread
- Only the within-subgroup variation
Correct answer: The overall standard deviation of all collected data
Pp uses the overall standard deviation calculated from all data points, while Cp uses the short-term within-subgroup estimate.
Question 4: In a process capability study, what does the 'voice of the customer' refer to?
- The control limits derived from process data
- The specification limits set by customer requirements (Correct answer)
- The natural tolerance of the process
- The range of the measurement system
Correct answer: The specification limits set by customer requirements
The voice of the customer is represented by specification limits (USL and LSL) that define acceptable product boundaries.
Question 5: If USL = 50, LSL = 30, and the process mean is 38, which formula gives the correct Cpk?
- min[(50-38)/3σ, (38-30)/3σ] (Correct answer)
- max[(50-38)/3σ, (38-30)/3σ]
- (50-30)/6σ
- (50+30)/2σ
Correct answer: min[(50-38)/3σ, (38-30)/3σ]
Cpk is the minimum of the upper and lower capability ratios, reflecting the worst-case distance to either spec limit.
Question 6: A process exhibits a 'sawtooth' pattern on a control chart. This most likely indicates:
- Random common cause variation
- Over-adjustment or tampering with the process (Correct answer)
- A sudden shift in the process mean
- High measurement system variation
Correct answer: Over-adjustment or tampering with the process
A sawtooth (alternating high-low) pattern is a classic sign of over-adjustment, where operators correct perceived error and create opposite swings.
Question 7: What is the relationship between process spread and the specification width when Cp = 1.33?
- The process spread equals the specification width
- The process spread is 75% of the specification width (Correct answer)
- The process spread is 133% of the specification width
- The process spread exceeds the specification width by 33%
Correct answer: The process spread is 75% of the specification width
Cp = 1.33 means 6σ = specification width / 1.33, so the process uses about 75% of the available specification space.
SPC Process Variation & Capability 2
A process has a Cpk of 0.85.
What does this indicate about the process?