The fundamental concept is extreme intolerance of errors and strict adherence to practices that you employ to prevent errors. That is why talking of 6-Sigma in the context of quality control (code checking) misses the point entirely. What we are supposed to focus on is how we can eliminate all possible contributing factors that produce errors in the code (which is quality assurance) in all phases of the SDLC. This is easier to conceive in a manufacturing setting with repeatable tasks than in a software development project where the tasks are more or less unique, but the more code reusability we can introduce the better we adhere to the intent of 6-Sigma. For testing, we should then also introduce reusable script segments and libraries commensurate with the code libraries. I suggest you do a google search on 6-Sigma and learn from GE, Motorola, and other companies that have established major initiatives, and also some Japanese companies have published their efforts in English. Good luck.