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

six sigma
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
  • All processes have variability 

  • Variability has definable causes

  • Typically only a few causes are significant

  • If causes can be identified & understood, they can be controlled

  • Design must be robust to the effects of remaining process variations

6 sigma 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

  • This can also be denoted as DPMO - defects per million opportunities ; so 3.4 DPMO would denote a 6σ level performance 

  • 3.4 DPMO = 99.99666% = 6σ

Note : Read a primer on DMAIC , one of the most common methods used in doing six sigma projects