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对“六西格玛”的一点质疑

近距

“六西格玛”的管理理念最早起源于摩托罗拉。是一种值得追求的管理理念。宣传上可能
搞混了不良品率和偏差（出错）机会两个概念。误导读者认为企业应用“六西格玛”的管
理就等于实现了六西格玛质量。

“六西格玛”的管理理念有其先进性。

1。量化质量管理。
2。表示企业不断追求更高质量管理目标。


Comments on Six Sigma 

I am trying to clarify the issue which arises from the following 
article: 

First, we need to understand the Norma Distribution Table ( so-called 
Z value table ). The probability listed on the Normal Distribution 
Table is the distribution probability when Z is larger than the 
specific Z value ( or Sigma number ). I just list the a few in the 
following table as well as the corresponding DPMO which is widely used 
in GE:

Z value         Probability             Defects per Million Opportunities(DPMO) 
1.5             6.68E-2                 66800 
2.5             6.21E-3                 6210 
3.5             2.33E-4                 233 
4.5             3.40E-6                 3.4 

The Z value here is the long term target which GE wants to achieve. 
However, there is a shift of 1.5  between long term Z value and short 
term Z value, or Z(long-term)=Z(short term)-1.5. To convert the above 
long term Z value to short term Z value, we have the following table:

Z(short-term)   Z(long-term)            DPMO 
3               1.5                     66800 
4               2.5                     6210 
5               3.5                     233 
6               4.5                     3.4 

When we talk about 6 Sigma, we are talking about the short-term  Z 
value and the actual long-term Z value is only 4.5. Hope this can help 
to understand Six Sigma meaning. 

In general people tend to solve problems and make decisions based on 
past experience or intuition instead of data. Six Sigma just provides 
some tools to help people to collect data and make the decision based 
on the data. In addition, it also provides a common language to 
communicate for all people with different backgrounds. It is a 
collection of some simple statistics knowledge but proves very helpful.

Regards, 

Gao Ming 

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