iSixSigma

The goal of Six Sigma is to increase profits by eliminating variability, defects and waste that undermine customer loyalty.

Six Sigma can be understood/perceived at three levels:

1. Metric: 3.4 Defects Per Million Opportunities. DPMO allows you to take complexity of product/process into account. Rule of thumb is to consider at least three opportunities for a physical part/component - one for form, one for fit and one for function, in absence of better considerations. Also you want to be Six Sigma in the Critical to Quality characteristics and not the whole unit/characteristics.
2. Methodology: DMAIC/DFSS structured problem solving roadmap and tools.
3. Philosophy: Reduce variation in your business and take customer-focused, data driven decisions.

Six Sigma is a methodology that provides businesses with the tools to improve the capability of their business processes. This increase in performance and decrease in process variation leads to defect reduction and vast improvement in profits, employee morale and quality of product.

Here's an article with more detail on defining Six Sigma: What is Six Sigma?

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Six Sigma is a rigorous and a systematic methodology that utilizes information (management by facts) and statistical analysis to measure and improve a company's operational performance, practices and systems by identifying and preventing 'defects' in manufacturing and service-related processes in order to anticipate and exceed expectations of all stakeholders to accomplish effectiveness.