Six Sigma Fundamentals and DMAIC
Six Sigma is a structured, data-driven approach to reducing variation and defects. Its central idea is that process output varies, and that variation is the enemy of quality. Sigma level expresses process performance as defects per million opportunities (DPMO); higher sigma means fewer defects. Candidates should know the rough benchmarks: 3 sigma is roughly 66,800 DPMO, 6 sigma is about 3.4 DPMO. You do not need to compute sigma from scratch, but you should understand that reducing variation and centring the mean both improve the sigma level.
DMAIC is the improvement cycle for existing processes: Define (problem, scope, customer requirements, business case), Measure (baseline data, measurement system validity), Analyse (root causes, not symptoms), Improve (pilot and implement solutions), Control (standardise, monitor, hand over). DMADV/DFSS applies to designing new processes. A common trap is placing activities in the wrong phase: a control chart is built in Control, a fishbone in Analyse, a baseline capability study in Measure.
Voice of the Customer feeds Critical to Quality requirements, which must be measurable. A SIPOC diagram sits at the Define boundary. Remember that Six Sigma uses both statistical tools and soft skills; it is not only about statistics.
Worked example: a team is told 'our invoice error rate is too high.' In Define they write a problem statement with baseline, scope, and impact. In Measure they verify the defect definition and collect a baseline. In Analyse they use a fishbone and data to find the dominant cause. In Improve they pilot a fix. In Control they add a check and a chart. If a question asks where to validate the measurement system, the answer is Measure.