Lean Six Sigma Yellow Belt Certification Statistical Thinking and Variation 2 — Questions and Answers
Question 1: Control charts have control limits set at ±3 standard deviations from the mean because:
- It is a regulatory requirement in manufacturing
- This range captures approximately 99.73% of common cause variation (Correct answer)
- Customer specifications typically fall at ±3 sigma
- Three is a convenient round number for calculations
Correct answer: This range captures approximately 99.73% of common cause variation
Setting control limits at ±3 standard deviations captures 99.73% of natural process variation, making points outside the limits a strong signal of special causes.
Question 2: What is the key difference between control limits and specification limits?
- Control limits are wider; specification limits are narrower
- Control limits are set by customers; specification limits are calculated from process data
- Control limits are calculated from process data; specification limits are set by customer requirements (Correct answer)
- They are different names for the same concept
Correct answer: Control limits are calculated from process data; specification limits are set by customer requirements
Control limits reflect what the process actually does (derived from data), while specification limits reflect what the customer requires — they measure entirely different things.
Question 3: A point falling outside the control limits on a control chart most likely indicates:
- The process is performing exceptionally well
- Common cause variation is present
- A specification limit has been violated
- A special cause of variation has occurred (Correct answer)
Correct answer: A special cause of variation has occurred
A point outside the control limits is a statistical signal that something unusual — a special (assignable) cause — has likely affected the process.
Question 4: A histogram is used in Six Sigma primarily to:
- Track data over time and identify trends
- Display the frequency distribution of process data (Correct answer)
- Show the relationship between two variables
- Rank defect categories by frequency
Correct answer: Display the frequency distribution of process data
A histogram displays how often values fall within defined intervals, revealing the distribution shape, central tendency, and spread of process data.
Question 5: What does a run chart help a Yellow Belt team identify?
- The frequency of each defect type in a process
- How two process variables are related
- Trends, shifts, or patterns in process data over time (Correct answer)
- The capability of a process relative to specifications
Correct answer: Trends, shifts, or patterns in process data over time
A run chart plots data sequentially over time, helping teams detect non-random patterns such as trends, shifts, or cycles that may indicate special causes.
Question 6: If a control chart shows a consistent upward trend of seven consecutive points, this is:
- Normal common cause variation that requires no action
- A signal of a special cause that should be investigated (Correct answer)
- Proof that the process is meeting specifications
- An indication that control limits should be recalculated
Correct answer: A signal of a special cause that should be investigated
Seven or more consecutive points trending in one direction is a recognized rule signaling special cause variation, even if none fall outside the control limits.
Question 7: The Voice of the Process (VOP) represents:
- Customer requirements expressed in measurable terms
- What the process is actually capable of delivering based on its own performance data (Correct answer)
- Management's targets for process improvement
- Employee feedback on process design
Correct answer: What the process is actually capable of delivering based on its own performance data
VOP describes a process's actual capability and behavior as revealed by data, in contrast to VOC (Voice of the Customer) which represents external requirements.
Lean Six Sigma Yellow Belt Certification Statistical Thinking and Variation 2
Control charts have control limits set at ±3 standard deviations from the mean because: