Six Sigma - A Breakthrough StrategyFICCI
CE
Sigma
Standard deviation is also referred to as the Greek letter sigma in quality management literature. Consequently process variation can be described by how many sigmas ( i.e how many multiples of standard deviation or SD ) fit within the specification limits.The higher this number, the better is the quality and vice versa.
Sigma is a statistical measure of the quality consistency for a particular process / product.
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Sigma process
From a sigma process we come to know that at what distance, in terms of the standard deviation, the specification limits are placed from the target value.
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Move beyond 3 sigma
Until a few years back all statistical process controls were designed on the basis of 3 sigma limits. It is only in the recent years that this concept has been challenged and the organizations have started looking for quality levels beyond 3 Sigma.
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Mean=8
SD = 2
Note : LSL is Lower specification limit and USL is Upper specification limit.
- 3 SD
+ 3 SD
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Precision
Lesser the standard deviation of the process, more precise or consistent is the process
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3 Sigma Vs 6 Sigma
The goal of Six Sigma program is to reduce the variation in every process to such an extent that the spread of 12 sigmas i.e. 6 Sigmas on either side of the mean fits within the process specifications. The figure on next slide shows what this looks like.
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LSL
USL
3 Sigma Vs 6 Sigma
In a 3 sigma process the values are widely spread along the center line,
showing the higher variation of the process. Whereas in a 6 Sigma
process, the values are closer to the center line showing
less variation in the process.
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3 Sigma Vs 6 Sigma
The comparison on the previous slide also shows a process in which the process spread of Six Sigmas (ie. 3 Sigmas on either side just fits within the specifications). In this case one must be extremely careful to ensure that the process average never slips off the target, otherwise the curve will shift and non-conforming items will increase. With Six Sigma requirement the process mean can shift by as much as 1.5 sigma before the likelihood of non-conforming items is increased. Even if the process mean does shift off center by as much as 1.5 sigma, only 3.4 non-conforming items per million parts should result.
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LSL
USL
*Parts Per Million
Sigma