As a professional, especially a professional in manufacturing industry, you may have heard, intentionally or unintentionally, a lot about Six Sigma. Sometimes it even gets annoying as it is over promoted by some organizations as a catholicon to any problems, which sounds suspicious. So what does Six Sigma mean exactly? Which kind of problems can be solved by it? What are the strengths and, of course, limitations of it?
Originally, Six Sigma, which equals to 6 σ, was a statistical concept initiated by Motorola in 1980s. It means the width of 6 times of standard deviations on both sides of the mean value, which is the width of 12 times of standard deviations in total, perfectly fits in the specification limits. This also means a quality level, which is 3.4 PPM in a long term. By the way, in statistics σ is the notation for standard deviation of the population and μ is for mean value.
To make it more sensible, let’s look at an example. Say, you would like to know the performance of a turning machine processing the outer diameter of a cylinder, so you take a random sample of 100 outputs from that machine. We know that the LSL (lower specification limit) and USL (upper specification limit) of outer diameter are fixed according to the drawing, so the more stable the process is, the smaller the standard deviation of the sample is, thus the more standard deviations can fit in the space between LSL and USL, and then the outer diameter of more parts will fall within specification, which means less defectives.
在实践中，六西格玛也意味着一种流程优化的方法论，这个方法论是19世纪90年代由通用电气公司发展出来的，也是现在六西格玛最广为人知的含义。这个方法论中主要涉及到两个系统地解决问题的途径，分别是 DMAIC 和 DMADV, 二者各自包含了五个按照严密的逻辑顺序排列的不同阶段。DMAIC指的是定义，测量，分析，改进，控制，主要用来解决现有流程存在的问题，而DMADV指的则是定义，测量，分析，设计，验证，主要用来杜绝新产品或流程可能出现的问题。
Practically, Six Sigma also means a methodology for process optimization, which was developed by General Electric in 1990s and more widely referred to in practice nowadays. Two major approaches involved in this methodology are DMAIC and DMADV, each of which stand for a systematic approach of problem solving with 5 different phases in a logical sequence. DMAIC, namely, stands for define, measure, analyze, improve, and control, aiming at solving problems on an existing process. DMADV, on the other hand, stands for define, measure, analyze, design, and verify, aiming at avoiding potential problems for a new product or a now process.
Although both approaches sound logical and easy to understand, each single phase has its own work package including a sufficient number of professional tools, which link to much more workload than a simple word sounds like. Six sigma projects, therefore, are projects that follow either of these two approaches to solve or avoid problems, and the project leaders are so called “belts”. The level these belts, usually distinguished by different colors (white, yellow, green, black), are certified by professional organizations according to one’s knowledge level, skillset, projects’ professional degree, and so on.
另外，当六西格玛在集团层面被提及时，无论是在运营领域还是商业领域，它还可以表示一种持续改进的企业文化，或者至少是这种文化的一部分。在这一点上，六西格玛往往会被看作是和Kaizen, Lean 之类并列的持续改善工具。
Additionally, when Six Sigma is mentioned at a corporate level, either in operational areas or commercial areas, it is always also referred as a corporate culture of continuous improvement, or at least part of that culture. For that matter, Six Sigma is usually considered as a tool for continuous improvement, which is pretty much similar with Kaizen, lean, and so on.
Companies promote these tools by building up relevant organizations, deploying trainings, providing coaching, and tracking projects, to create such a culture, where employees have the awareness and capability to identify problems, to find the right method for each problem, and to solve them with success eventually. And Master Black Belt, is the person who is responsible to promote Six Sigma at a corporate level.