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.
FICCI
CE
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.
FICCI
CE
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.
FICCI
CE
Mean=8
SD = 2
Note : LSL is Lower specification limit and USL is Upper
specification limit.
- 3 SD
+ 3 SD
FICCI
CE
Precision
Lesser the standard deviation of the process, more precise or
consistent is the process
FICCI
CE
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.
FICCI
CE
2
3
4
5
6
7
8
9
12
10
16
15
14
13
11
1
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.
FICCI
CE
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.
FICCI
CE
2
3
4
5
6
7
8
9
12
10
16
15
14
13
11
1
LSL
USL
*Parts Per Million
Sigma