SPC Fundamentals 3 — Questions and Answers
Question 1: What is the difference between Cp and Cpk?
- Cp measures long-term capability; Cpk measures short-term capability
- Cp ignores centering; Cpk accounts for both spread and centering (Correct answer)
- Cp is used for attribute data; Cpk is used for variable data
- Cp uses specification limits; Cpk uses control limits
Correct answer: Cp ignores centering; Cpk accounts for both spread and centering
Cp only measures whether the process spread fits within specifications, while Cpk also accounts for how well the process is centered between the limits.
Question 2: Which of the following is NOT one of Walter Shewhart's two types of variation?
- Chance causes (common causes)
- Assignable causes (special causes)
- Structural causes (Correct answer)
- Both B and C
Correct answer: Structural causes
Shewhart identified chance causes (common) and assignable causes (special); 'structural causes' is not part of his framework.
Question 3: A process producing 3.4 defects per million opportunities operates at what sigma level?
- 3 sigma
- 4 sigma
- 5 sigma
- 6 sigma (Correct answer)
Correct answer: 6 sigma
Six Sigma quality is defined as 3.4 DPMO, which accounts for a 1.5-sigma long-term shift in the process mean.
Question 4: Which type of data would you use an Individuals and Moving Range (I-MR) chart for?
- Attribute data from large subgroups
- Count data such as number of defects
- Continuous data collected one observation at a time (Correct answer)
- Proportion nonconforming from inspection lots
Correct answer: Continuous data collected one observation at a time
The I-MR chart is designed for continuous variable data where only one measurement is produced at each time point.
Question 5: The term 'voice of the process' (VOP) in SPC refers to:
- Customer feedback on product quality
- What control charts reveal about process behavior (Correct answer)
- Engineering specifications set by designers
- Management targets for production output
Correct answer: What control charts reveal about process behavior
VOP refers to what the data and control charts tell us about how the process is actually performing.
Question 6: If a process has Cpk = 1.0, what percentage of output is expected to be within specification?
- 95.44%
- 99.73% (Correct answer)
- 99.9937%
- 68.27%
Correct answer: 99.73%
A Cpk of 1.0 corresponds to the process mean being exactly 3 sigma from the nearest specification limit, yielding 99.73% within spec.
Question 7: Which SPC tool is used to identify and prioritize the most significant causes of a quality problem?
- Control chart
- Pareto chart (Correct answer)
- Scatter diagram
- Histogram
Correct answer: Pareto chart
A Pareto chart ranks defect categories by frequency, helping teams focus on the 'vital few' causes that account for most problems.
SPC Fundamentals 3
What is the difference between Cp and Cpk?