Six Sigma 之来由

要了解Six Sigma这个名词的来由，就必须由Sigma（）这个字谈起。相信很多人都知道，在数学里，Sigma其实就是我们常说的Standard Deviation。

以当今的科技，无论你使用多么精准的机械，始终无法连续制造出一摸一样的成品。无可否认，很多成品外表看起来都很相似，但是，如果经过详细的测量，我们会发现到所有的产品都有一些小小的偏差。这些偏差或许可以小得令我们很不在呼，可是，很多时候这些小小的偏差对整个成品生产过程带来诸多的问题。

在现实的世界里，偏差是无法避免的。在成品生产过程当中，我们需要原料，机械及人力。原料与原料之间的差异，机械与机械之间的差异以及人与人之间的差异，造成了一个不完善的制成品。偏差永远存在，无可避免，唯一能做的，就是想尽办法减少差异。

Six Sigma的意思是在某个成品的Standard Deviation 相等于( UCL – mean ) 或是 ( mean – LSL ) 的六分之一，我们能够以为该成品制造过程的能力已达到Six Sigma的标准。也能够合理的推测，在百万个制成品当中，最多只有3.4个不合规格的成品。所以，想要达致Six Sigma的标准，并不是一项简单的工程了。

Sigma (the lower-case Greek letter σ) is the symbol represents standard deviation of a statistical population. It is not new to peoples who have some basic statistic study.

Based on today’s technology, we are still not possible to reproduce exactly alike regardless how high precision the equipment we use to produce the part. No doubt, in many products, there are looks alike from the outside, but if you go through a detail measurement, you will notice there are deviation, the deviation could be very minor to the extend that peoples used to ignore, but most of the time, it is the root cause of many problems that we face in our daily life.

In the real world, deviation is unavoidable. Deviation exists due variation between material batches, machine to machine and peoples. The deviation cause an imperfect product. Since we can avoid deviation, what we can do, is to think way to minimize it.

A Six Sigma process mean when a process standard deviation is 1/6 of the range between process mean and upper control limit or lower control limit. Statistically, there are 3.4 chance for Six Sigma process to produce a defect out of one million part produced. It looks simple, but in fact it is not so simple to achieve Six Sigma level.