Six Sigma
Measure, reduce variation & improve
What is six sigma ?

a statistical measure of process capability

a set of tools and techniques for reducing variation

a philosophy about operational excellence

A Measurement Scale which compares the output
of a process to customer requirements
Its all about continuous improvement by reducing variation
What is variation

as a customer you want consistency in any product or service that you consume

variation causes dissatisfaction or defect

defect  any thing that doesnt meet customer expectation

we generally measure process performance as an average v/s some target

average would hide the variation between data points

example  average of 4 cars' speed 25, 50 , 75 & 100 is 62.5 , does this show or hide the difference between each car ?

So variation should be measured

we measure variation via (depending on the data distribution)

Standard deviation

Range

Inter quartile range

Basic premises underlying six sigma
If you cant measure it, you can't manage it
6 Sigma as a measure of performance
6 sigma = 6 σ ; σ is Greek letter s
σ denotes standard deviation  average distance of data points in a set from the mean (average)
Standard deviation shows variation from mean & your mean should be near or on the target
Smaller the standard deviation ; lesser the variation
So if your data sets' variation, denoted by σ, is small enough, you would be able to put 6 σ (six std deviations) between the target and customer specifications
All processes have variability
Variability has definable causes
This can also be denoted as DPMO  defects per million opportunities ; so 3.4 DPMO would denote a 6σ level perfomance
Typically only a few causes are significant
If causes can be identified and understood, they can be controlled
Design must be robust to the effects of remaining process variations
3.4 DPMO = 99.99666% = 6σ
Note : Read a primer on DMAIC , one of the most common methods used in doing six sigma projects