Six Sigma

Six Sigma is a structured process-improvement approach commonly using Define, Measure, Analyze, Improve, and Control. It seeks dependable performance against requirements, using measurement and analysis of defects and variation.

Six Sigma organizes improvement around a defined requirement, trustworthy measurement, analysis, a tested change, and ongoing control. Variation matters when it causes a process to miss a relevant requirement. Reducing variability around the wrong target can make a process consistently unsatisfactory, so customer needs and the process mean matter alongside spread.

The familiar 3.4 defects per million figure uses a particular convention involving a 1.5-standard-deviation mean shift; it is not the unqualified tail probability beyond six standard deviations of a centered normal distribution. Actual capability assessment needs a stable process, suitable distributional assumptions, and a defensible definition of a defect opportunity.

When to use it

When process inconsistency is producing quality problems; when distinguishing between systemic and specific causes of variation would improve intervention; when measurement-driven quality improvement is needed; when the balance between standardization and creativity needs calibration.

How it can help

Define what good performance means, verify measurements, assess stability, investigate causes, test improvements, and monitor their effect. Prioritize causes using evidence rather than assuming an 80/20 split.