Having recently earned my Six Sigma green belt from IISE (Institute of Industrial and Systems Engineers), I can say that my initial impression is that it is a very useful skill to have in industry. Six Sigma centers around DMAIIC (define, measure, analyze, improve, implement, control) as Hiren Rana previously touched upon, which are the basic steps to lessening the the variation of a process. As a greenbelt holder in Six Sigma, the general involvement is the getting involved with system improvement in an industrial setting as part time work alongside their full time job (working under the supervision of a Black Belt). Black Belt and master black belts normally specialize exclusively in Six Sigma process improvement activities as their full time careers and are more extensively trained in Six Sigma (100+ hours more class time).

One of the disadvantages of Six Sigma is that of over implementation (obsession) to a fault. This involves too tightly controlling a company’s creative processes caused by a resulting bureaucracy environment. In addition, a company can overemphasize this process to create a process with so little variation but at an exponentially higher cost. A more cost effective alternative would have been to allow for slightly more variation at a drastically reduced cost. In fact Six Sigma recognizes this flaw and devotes a chapter to analyzing to what degree a process should be controlled before it becomes no longer worth it to the company.

http://smallbusiness.chron.com/advantages-amp-disadvantages-six-sigma-43963.html