Before I discuss Lean Six Sigma, I'll quickly cover the fundamentals of the two contributing approaches, Six Sigma and Lean.
Six Sigma is typically defined as a disciplined, statistical approach aimed at increasing profitability by reducing defects. There is a large Six Sigma "tool box" of analytical, data-driven approaches that help companies improve quality in this way.
I tend to think about Six Sigma more holistically than many of its current adherents. I also take to heart a comment made by Bill Smith, the Motorola engineer who invented Six Sigma: “If you want to improve something, involve the people who are doing the job.”
So I see the power of Six Sigma as greatest when it is thought of more broadly, as an organizational shift wherein employees use a disciplined approach to improve overall business performance, using data and focusing on controlling process variation. This is even more powerful when built into a Strategy Execution approach, so that improvement efforts are focused on the most strategically important problems for the business.