Six Sigma without Lean means you lose the Lean Tools. You still have a robust process improvement method but you don’t get instruction on how to conduct things like Process Walks and those provide a way to understand the process, engage people and build a detailed map. It would be unfortunate to lose tools like that. There are also a lot of Lean concepts that come into play in the Improve Phase that are classic solutions like Cell Design, Kanbans, etc. We prefer the combination of Lean and Six Sigma so we get the entire tool kit!