Lean Six Sigma Methodology
Domain 2 covers the Lean Six Sigma methodology. The exam tests the DMAIC phases and their deliverables, when DMADV/DFSS is used, and how Lean tools fit into a data-driven project.
Core method
- DMAIC improves an existing process: Define (charter, VOC, SIPOC, CTQ, scope), Measure (data-collection plan, measurement-system analysis, baseline), Analyze (root causes using Pareto, 5 Whys, cause-and-effect, FMEA), Improve (pilots, mistake-proofing, flow, standard work), Control (control plan, SPC, response plan, handover).
- Each phase ends with a tollgate: deliverables are reviewed before the next phase.
- DMADV (Define, Measure, Analyze, Design, Verify) is for a new product or process, or when redesign is needed because the existing process cannot meet CTQs.
- Lean tools support DMAIC: value-stream mapping, 5S, kaizen, kanban/pull, takt time, and poka-yoke.
- The charter states the problem, goal, scope, business case, team, and milestones; it prevents scope creep.
Worked example
A billing team processes 3,000 invoices monthly with 120 errors, a 4% error rate. In Define, the charter sets a goal to cut errors by 50% in 90 days and limits scope to manual invoices. In Measure, the team verifies that two reviewers classify errors the same way before accepting the baseline. In Analyze, a Pareto shows 70% of errors are missing purchase-order numbers; 5 Whys traces the cause to manual entry. In Improve, a required-field check (poka-yoke) and standard work are piloted in one region; errors fall to 45 in the same volume, a 62.5% reduction. In Control, a weekly control chart and owner response plan sustain the gain.
Traps
- Jumping to solutions before Define and Measure are complete.
- Confusing DMAIC with DMADV.
- Measuring without validating the measurement system.
- Treating a symptom as a root cause.
- Treating Control as paperwork only; it requires monitoring and a response plan.