Six Sigma Fundamentals and DMAIC
What the exam tests
This domain covers the Six Sigma logic: variation causes defects, projects should be selected on business impact, and DMAIC is a gated problem-solving method. Expect sigma level, DPMO, cost of poor quality, project roles, and the purpose of each phase.
Core rules
- DMAIC: Define the problem and charter; Measure the baseline and measurement system; Analyze root causes; Improve with validated solutions; Control and standardize.
- Sigma level expresses defects per million opportunities. The common 1.5-sigma shift accounts for long-term drift; compare sigma levels only when the shift convention is stated.
- Roles: Champion sponsors and removes barriers; Master Black Belt coaches; Black Belt leads cross-functional projects; Green Belt leads projects within their area.
- Y = f(x): the output Y is a function of process inputs x. DMAIC finds and controls the critical few x values.
- Tollgates are decision points: deliverable evidence should be reviewed before the next phase begins.
Worked example
A process has 4 defects across 10,000 opportunities: DPMO = 4 / 10,000 x 1,000,000 = 400. The long-term tail area of 0.0004 corresponds to z of about 3.35. Adding the 1.5 shift gives a sigma level of about 4.85, not 6. The reasoning: DPMO is converted to a z-value, then the convention is applied. If you skip the shift, you report 3.35 and the answer is wrong under the usual Six Sigma convention.
Traps
Reporting PPM as DPMO without counting opportunities; applying the 1.5 shift in one question and not the next; treating DMAIC as strictly linear when loops back are normal; and jumping to Improve before the Measure baseline is credible.