Lean Six Sigma Black Belt Practical
The published domain structure for this EXIN LSSA Black Belt Practical exam is not split into named subdomains, so this section treats the exam as one integrated practical domain: applying DMAIC to a realistic process problem and defending each decision with data. Expect scenario questions that ask what to do next, which tool is valid, or why a claimed improvement is not yet proven.
Core rules and concepts
- Define: convert the problem into a charter with problem statement, goal, scope, business case, team, and tollgate dates. Validate scope with SIPOC and translate VOC into measurable CTQs.
- Measure: write operational definitions and a data-collection plan before collecting data. Run MSA first: for continuous data, a Gage R&R %GRR under 10% is generally acceptable, 10–30% conditional, and above 30% needs action; for attribute data, check agreement and kappa.
- Baseline and capability: establish a stable baseline before quoting capability. Cp compares specification width with process spread; Cpk also accounts for centering. For non-normal data, use an appropriate capability method rather than forcing Cp/Cpk.
- Analyze: use process maps, Pareto, FMEA, and hypothesis tests. Check assumptions, distinguish statistical from practical significance, and confirm causes with data rather than opinion.
- Improve: screen causes, pilot changes, and verify results against the baseline. Use DOE when several factors interact; randomize and block where appropriate.
- Control: transfer the improved process with a control plan, SPC where justified, standard work, poka-yoke, a response plan, and benefit validation.
Worked example
An invoice process has 8% errors; the charter goal is below 3% in 90 days. Before baseline capability, the team checks attribute agreement and finds only 88% agreement on what counts as an error, so it rewrites the operational definition. A Pareto then shows 62% of errors are missing purchase-order references. The p-chart is stable, so the team investigates root causes, pilots a mandatory PO field, and obtains 2.8% errors. It does not declare victory until a control plan, weekly audit, and p-chart monitor hold the gain.
Traps that cost marks
- Starting improvement before MSA and a stable baseline.
- Quoting Cpk from an unstable process or ignoring non-normality.
- Treating p > 0.05 as proof that no effect exists.
- Confusing Cp with Cpk or reporting capability without specification limits.
- Closing the project without a control plan, response plan, or verified benefit.