My understanding of CMM is that it is not "prescriptive". It doesn't dictate exact methods or steps to follow, but instead provides a set of best practices and maturity levels. It is "DIAGNOSTIC" and describes your level of maturity. Six Sigma is an advanced data driven, closed loop, TOOL/method that could be applied at CMM Level 4 (Quantitative) or above. Six Sigma is one of many actual methods to improve performance. An entity would have no success implementing Six Sigma if they are at CMM Level 1, 2, or even 3 because the Numerical Data maturity level for key processes has not been achieved.