Lean Six Sigma Green Belt Certification Lean Six Sigma Green Belt Control Phase and Sustainment 1 — Questions and Answers
Question 1: In the Control phase, what is the primary function of a control chart's Upper Control Limit (UCL)?
- To define the customer's maximum acceptable specification
- To signal when a process has shifted beyond natural variation (Correct answer)
- To set the target mean for the process
- To replace the need for a reaction plan
Correct answer: To signal when a process has shifted beyond natural variation
The UCL (and LCL) on a control chart represent the boundaries of natural process variation (±3 sigma). A data point beyond the UCL signals a special-cause event requiring investigation, not just common-cause noise.
Question 2: Which of the following best describes a 'mistake-proofing' device implemented during the Control phase?
- A statistical process control chart updated weekly
- A physical or electronic mechanism that prevents a defect from occurring or escaping (Correct answer)
- A checklist completed by the operator at shift end
- A training document given to new employees
Correct answer: A physical or electronic mechanism that prevents a defect from occurring or escaping
Poka-yoke (mistake-proofing) devices are physical or automated mechanisms — like sensors, jigs, or interlocks — that either prevent an error from happening or detect it immediately. They are more robust than procedural controls like checklists.
Question 3: A Lean Six Sigma team has completed improvements and is closing the project. Which metric best indicates that gains have been sustained six months after handover?
- The project charter scope statement
- The process sigma level remaining at or above the improved baseline (Correct answer)
- The number of team members still engaged in the project
- The reduction in the number of control charts in use
Correct answer: The process sigma level remaining at or above the improved baseline
Sustained improvement is demonstrated by ongoing process performance data — specifically, the sigma level (or defect rate) remaining at or above the level achieved during the Improve phase, confirming the gains have held.
Question 4: What is the purpose of a 'lessons learned' document completed at the end of the Control phase?
- To document remaining defects for the next project team
- To capture knowledge and best practices so future projects can benefit from the team's experience (Correct answer)
- To formally close the process control plan
- To reassign team members to new roles
Correct answer: To capture knowledge and best practices so future projects can benefit from the team's experience
A lessons learned document captures insights — what worked, what didn't, and what would be done differently — enabling organizational learning and preventing future teams from repeating the same mistakes.
Question 5: When selecting a control chart type in the Control phase, which chart is appropriate for monitoring the proportion of defective units in a variable-size subgroup?
- X-bar and R chart
- p-chart (Correct answer)
- c-chart
- I-MR chart
Correct answer: p-chart
The p-chart monitors the proportion (fraction) defective and is designed for attribute data where subgroup sizes vary. The c-chart tracks count of defects per unit in fixed-size subgroups, while X-bar/R and I-MR charts are for continuous data.
Question 6: In the Control phase, what is the recommended response when a single data point falls outside the control limits on an SPC chart?
- Immediately recalculate the control limits to include the new point
- Ignore it if the next point falls back within limits
- Investigate for a special-cause, document findings, and follow the reaction plan (Correct answer)
- Shut down the process permanently until a new improvement cycle is run
Correct answer: Investigate for a special-cause, document findings, and follow the reaction plan
A point outside control limits signals a special-cause variation. The correct response is to investigate the root cause, document what was found, and execute the pre-defined reaction plan — not to recalculate limits or ignore the signal.
Lean Six Sigma Green Belt Certification Lean Six Sigma Green Belt Control Phase and Sustainment 1
In the Control phase, what is the primary function of a control chart's Upper Control Limit (UCL)?