Lean Six Sigma Green Belt Certification DMAIC: Improve and Control 3 — Questions and Answers
Question 1: A Lean Six Sigma team uses a 2^3 full factorial experiment with center points. What is the primary reason for adding center points?
- To increase the number of replicates for main effects
- To detect curvature in the response and estimate pure error (Correct answer)
- To reduce the number of experimental runs required
- To balance the design across blocking variables
Correct answer: To detect curvature in the response and estimate pure error
Center points are added to a factorial design to test for curvature (non-linearity) in the response surface and provide an estimate of pure experimental error.
Question 2: Which control chart is most appropriate for monitoring the fraction of defective units in large, variable-sized subgroups?
- p-chart (Correct answer)
- np-chart
- u-chart
- c-chart
Correct answer: p-chart
The p-chart tracks the proportion defective and accommodates varying subgroup sizes, unlike the np-chart which requires a constant subgroup size.
Question 3: During the Control phase, an operator notices a trend of seven consecutive points rising on an X-bar chart but all within control limits. What is the correct response?
- Continue monitoring; the process is in control because no points exceed the limits
- Investigate for a special cause because a run of seven in one direction is a non-random pattern (Correct answer)
- Recalculate the control limits to account for the trend
- Increase subgroup size to reduce false alarms
Correct answer: Investigate for a special cause because a run of seven in one direction is a non-random pattern
Western Electric rules recognize that seven consecutive points trending in one direction is a non-random signal of a special cause, even if all points remain within the three-sigma control limits.
Question 4: A team implements a 5S program as part of the Improve phase. Which pillar directly reduces search time by assigning a designated location to every item?
- Sort (Seiri)
- Set in Order (Seiton) (Correct answer)
- Shine (Seiso)
- Standardize (Seiketsu)
Correct answer: Set in Order (Seiton)
Set in Order (Seiton) assigns and visually marks a specific location for each item so workers can find and return it without searching.
Question 5: What statistical method is used to confirm that a process improvement resulted in a statistically significant change in the process mean?
- Control chart recalculation
- Hypothesis test (e.g., t-test or ANOVA) (Correct answer)
- Measurement System Analysis re-study
- Process capability index comparison
Correct answer: Hypothesis test (e.g., t-test or ANOVA)
A hypothesis test such as a two-sample t-test or ANOVA formally evaluates whether the observed difference in means before and after improvement is statistically significant.
Question 6: In a response surface methodology (RSM) experiment, what is the purpose of a 'steepest ascent' path?
- To identify factor levels that minimize experimental error
- To move efficiently toward the region of optimal response between screening and optimization runs (Correct answer)
- To confirm that the model has a saddle-point configuration
- To replicate center points at the boundary of the design space
Correct answer: To move efficiently toward the region of optimal response between screening and optimization runs
The method of steepest ascent uses the gradient of a first-order model to guide sequential experiments toward the optimal region before fitting a higher-order surface.
Question 7: A Standard Operating Procedure (SOP) is revised after a process improvement. Which action best ensures operators consistently follow the updated SOP?
- Email the SOP to all team members
- Post the SOP in the team SharePoint folder
- Conduct documented training and verify competency through observation (Correct answer)
- Translate the SOP into a flowchart only
Correct answer: Conduct documented training and verify competency through observation
Documented training with competency verification confirms that operators understand and can perform the updated procedure, closing the gap between having documentation and actually executing it.
Lean Six Sigma Green Belt Certification DMAIC: Improve and Control 3
A Lean Six Sigma team uses a 2^3 full factorial experiment with center points.
What is the primary reason for adding center points?