Continuous Improvement and Lean Six Sigma
What the exam tests
DMAIC/DMADV structure, lean tool selection, waste identification, sigma-level and DPMO conversion, and the statistical tests used to prove improvement.
Core rules
- DMAIC applies to existing processes; DMADV (or IDOV) applies to new designs. Tollgates require deliverables before the next phase begins.
- Lean defines value from the customer's perspective and removes the eight wastes: defects, overproduction, waiting, non-utilized talent, transportation, inventory, motion, excess processing (DOWNTIME).
- Takt time = available production time / customer demand. OEE = availability x performance x quality. Value stream mapping shows current and future state; 5S, SMED, kanban, jidoka and poka-yoke are countermeasures, not goals.
- Six Sigma quality is 3.4 defects per million opportunities, derived from a 6 sigma process with the conventional 1.5 sigma long-term shift. DPMO = (defects / (units x opportunities)) x 1,000,000.
Worked example
A cell has 27,000 available seconds per shift and demand of 450 units, so takt time is 60 seconds. Actual cycle time is 72 seconds, giving capacity of 27,000/72 = 375 units, a shortfall of 75 units per shift. Separately, 5 defects across 2,000 units with 4 opportunities each means 8,000 opportunities, so DPMO = 625, a short-term Z of about 3.23 and a reported sigma level near 4.7. Both numbers drive the project charter.
Traps that cost marks
Quoting sigma level without stating the 1.5 shift convention. Counting opportunities inconsistently between projects, which makes DPMO incomparable. Running a t-test or ANOVA without checking normality, independence and equal variance. Deploying lean tools before defining value. Improving capability without a control plan, so gains decay. Confusing takt time with cycle time.