Six Sigma Basic Overview

What is Sigma?

Sigma σ (lower case) or  is the 18th Character in the Greek alphabet and it is equivalent to the English language ‘s’

In statistics, σ symbol is used for standard deviation and is a measure that is used to quantify the amount of variation or dispersion of a set of data values.

The Greek letter σ (sigma) marks the distance on the horizontal axis between the mean, µ, and the curve’s inflection point.

The greater this distance, the greater is the spread of values encountered. For the green curve shown above, µ = 0 and σ = 1.

The upper and lower specification limits (USL and LSL, respectively) are at a distance of 6σ from the mean. Because of the properties of the normal distribution, values lying that far away from the mean are extremely unlikely.
Even if the mean were to move right or left by 1.5σ at some point in the future (1.5 sigma shift, coloured red and blue), there is still a good safety cushion. This is why Six Sigma aims to have processes where the mean is at most 6σ away from the nearest specification limit.

What is Six Sigma?

Six sigma terminology originated with statistical modelling of manufacturing processes. This term basically defines quality which is almost near the perfection limit. It is a data driven approach which helps in eliminating defects (driving toward six standard deviations between the mean and the nearest specification limit).

It is said that if there are not more than 3.4 defects per million units, then six sigma has been achieved. The main idea behind the six sigma concept is process  improvement and reduction of variation.

The Six Sigma DMAIC process (define, measure, analyze, improve, control) is an improvement system for existing processes falling below specification and searching for incremental improvement.

The Six Sigma DMADV process (define, measure, analyze, design, verify) is an improvement system which is used to develop new processes or products at Six Sigma quality levels.

The Six Sigma Management System drives clarity around the business strategy and the metrics that most reflect success with that strategy. It is rightly said that –

“It provides the framework to prioritize resources for projects that will improve the metrics, and it leverages leaders who will manage the efforts for rapid, sustainable, and improved business results”


The DMAIC project methodology has five phases:

  • Define the system, the voice of the customer and their requirements, and the project goals.
  • Measure key aspects of the current process and collect relevant data.
  • Analyze the data to investigate and verify cause-and-effect relationships. Determine what the relationships are and attempt to ensure that all factors have been considered. Seek out root cause of the defect under investigation.
  • Improve or optimize the current process based upon data analysis using techniques such as design of experiments, mistake proofing, and standard work to create a new, future state process.
  • Control the future state process to ensure that any deviations from target are corrected before they result in defects. Implement control systems such as statistical process control, production boards, visual workplaces, and continuously monitor the process.
    Some organizations add a Recognize step at the beginning, which is to recognize the right problem to work on, thus yielding an RDMAIC methodology.


The DMADV project methodology, known as DFSS (“Design For Six Sigma”), features five phases:

  • Define design goals that are consistent with customer demands and the enterprise strategy.
  • Measure and identify CTQs (characteristics that are Critical To Quality), product capabilities, production process capability and risks.
  • Analyze to develop and design alternatives
  • Design an improved alternative, best suited per analysis in the previous step
  • Verify the design, set up pilot runs, implement the production process and hand it over to the process owner(s)

Implementation roles

Executive leadership from the top level management is the main requirement to ensure the successful implementation of the six sigma concept in any organization. There are different categories in six sigma:

Master Black Belts

They are identified by champions, act as in-house coaches on Six Sigma. They devote 100% of
their time to Six Sigma. They assist champions and guide Black Belts and Green Belts. Apart from statistical tasks, they spend their time on ensuring consistent application of Six Sigma across various functions and departments.

Black Belts

They operate under Master Black Belts to apply Six Sigma methodology to specific projects. They
primarily focus on Six Sigma project execution and special leadership with special tasks, whereas Champions and Master Black Belts focus on identifying projects/functions for Six Sigma.

Green Belts

They are the employees who take up Six Sigma implementation along with their other job responsibilities, operating under the guidance of Black Belts.

Yellow Belts

Some organizations use additional belt colors, such as Yellow Belts, for employees who have basic training in Six Sigma tools and generally participate in projects and “White belts” for those locally trained in the concepts but do not participate in the  project team. “Orange belts” are also mentioned to be used for special cases.

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