An Introduction to Six Sigma
It embraces a number of concepts, but is grounded heavily in statistical theory (eg: 'sigma' represents standard deviation in a population), with a six sigma process often considered to be one that produces '3.4 defective parts per million opportunities'.
It employs two methodologies, known as DMAIC (Define, Measure, Analyze, Improve and Control) and DMADV (Define, Measure, Analyze, Design, Verify). Having stated this, there are a number of other closely related variants.
Six Sigma also identifies a number of 'roles' within the organization, which it considers to be critical to the success of implementation. Broadly, these are: Executive Leadership, Champions, Master Black Belts, Experts, Black Belts and Black Belts.
represent standard deviation (a measure of variation) of a population (lower-case 's', is an estimate, based on a sample).