Re: SIX SIGMA

[walker@wawa.demon.co.uk (Walker Associates)]

Here are a few points on Six Sigma which reflect my understanding of
the basic concept.

1. A six sigma process is one where the specification width is plus or
minus six sigma. Ie the Process Capability Cp = 2

2. A "worst case" assumption is made that the process is off centered
by 1.5 sigma. Ie the process Capability Cpk = 1.5

3. Based on assumption 2 you then calculate (using one tail) that the
defect rate is 3.4 parts per million.
Clearly this calculation is flawed on two counts
i. The 1.5 sigma off set assumption
ii. The conformance of any actual distribution to the normal
distribution out at six sigma is suspect

4. From a Deming viewpoint the approach is flawed in the same way that
process capability is flawed. Ie it is based on specifications rather
than target values.

5. The control limit on a control chart provides us with an
operational definition of a Special Cause. Hence it is difficult to
understand why you should count defects with a six sigma process. The
assumption should always be a Special Cause when a defect occurs.

6. Six Sigma is also used "backwards" to calculate the process
capability from  a count of defects.. Here you have to asses the
"opportunities for defects" (eg No. solder joints in a product) and
the number of defects that occur ( eg. number of defective joints).
You then use the resulting parts per million defective to calculate
sigma.
 Here of course the assumption of a 1.5 sigma offset results in a
higher sigma value than you would otherwise expect.

7. In order to calculate the total number of defects that occur from
data which may be available such as first pass yield ( number of units
with no defects at all) the poisson distribution is assumed. 

8. The purpose in calculating sigma from defects is to create a common
measure for all processes where variables measures are not available.

9. I think the improvement methodology behind six sigma is much like
any other.
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