Lean/Six Sigma
In the simplest terms, Lean/Six Sigma is a comprehensive approach that relies heavily on the use of statistical tools to measure and improve process quality based on customer requirements. Lean typically focuses on the identification and removal of waste in processes. Six Sigma is most often used to establish Process Management, or to drive Process Improvement (Root Cause Problem Solving), and Process Design with a focus on a reduction in process variation and removal/avoidance of defects. 

Sigma levels can be calculated either by using Yield - defined as "good" units produced by a process the first time (i.e., no rework) or Defects Per Million Opportunities. (Six Sigma translates to 3.4 "defects" per million opportunities.) 
Benefits of using a Lean/Six Sigma approach to process measurement & improvement include:
1. Works in both manufacturing and service environments
2. Driven by customer requirements
3. Establishes a common language and set of tools for organizational measurement and process improvement
4. Supports the quantification of processes that seem difficult to measure
5. Addresses both waste and defect reduction

6. Basic techniques are trainable at all levels of the organization