Monday, June 17, 2013


I hope this diagram is clear..please click it to see an enlarged view.

Nothing has created so much ado as the Six Sigma Quality system or Six Sigma Quality control procedure. But everything said and done Six Sigma has its origin in basic statistics.

One of the earliest Statistical procedures is the Histogram ...drawn using Class Intervals and Frequency ultimately leading to an Ogive and the most interesting aspect of these curves is that they are hill-shaped curves.

Such curves have given rise to a Normal Distribution..and the shape of the curve suggests that in a random population it is Normal to see the data distributed around the mean as seen in this distribution which can be expressed mathematically as it comprises of a combination of exponentially increasing and decreasing functions.


What is a Standard  Normal  Curve..its area under the curve is 1 and has equal areas on either side of the Mean..this curve is what six sigma is based.Note the above indicates the area exterior to the sigma levels.

Simply said under 1 sigma the area exterior to the level is 31.74 % and the area interior to the 1 sigma level is 100-31.74 = 68.26 %.

One of the preliminary methods of implementing Six Sigma is bench marking a quality system just to understand at what level the system operates at any given point in time. If there are 10 defective products every 10,000 it translates to roughly 1000 defects per million or from the above table a 3.3 sigma level.Now the task is to boost it to a six sigma level.

There is an interesting fact about Six sigma in it that from the above table it can be inferred that at six sigma it translates to 2 defects per billion.....Billion is not accepted as an exchange rate or yardstick of measurement and hence the concept of 1.5 sigma shift..which is 4.5 sigma..where we get 3.4 defects per million..and this is the refernce along which six sigma works.

The best of the quality systems under normal circumstances may be working at 3 sigma which is 2700 defects per million an unacceptable level..which means that (100 - 0.27)   99.73 % of the products are with lower and upper tolerance level of quality and implementing six sigma simply means that increasing the peak of the current normal curve to the extent of bringing many more of the defective components lying beyond this area within the tolerance fold so that the defects would be so miniscule to the extent of 3.4 defects per million an acceptable quality standard for at least most manufacturing situations.

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