Six sigma (a Lean tool) is associated with DMAIC methodology which is a problem solving method where six sigma tools are used to analyze the process data and finally the root causes behind the deviations are identified.
You can go thru the following paper on Six sigma although it is referred to a different area. Hope it'll help little bit to figure your query out.
Dear Mrs rostami, II don't know anything about Sigma sex. I have worked upon DEA with real, fuzzy and stochastic data. I have many ISI paper and have worked DEA to rank the application of some organizations such as hospital, municipality and electricity distribution and others.
S.I. Salahelin and I. S. Abdelwahab (2009). Six Sigma Practices in the banking sector in Qatar. Global Business and Management Research Vol. 1(1) 23-35