What is Six sigma?

If as a beginner in Six Sigma journey you are confused about exactly what is Six Sigma, you have come to the right place!

Six Sigma is a tool, methodology, a metric, a measure, a benchmark, a goal, a philosophy and a statistical term.

Let us see, how does ‘Six sigma’ fit into all these roles?

As a statistical term: 6σ is a statistical term in which σ is a Greek letter which denotes standard deviation. Standard deviation represents the variation of the process or the process spread around mean. A Six sigma process is a process in which the 6 standard deviations above or below mean are within the nearest specification limits.


In the above figure, the orange curve has high spread, i.e. high standard deviation; while the blue curve has lower standard deviation as compared to the orange one. In general terms, we can say process having blue curve is better since it is more clustered around the mean.

Let’s also consider the two vertical black lines now, which are the upper and lower specification limits. In this case, the blue curve is well within the specification limits. If we also assume that the 6σ spread above and below the mean are within these specification limits, the blue process is a Six Sigma process. In a six sigma process, there is a scope of 1.5σ shift towards upper/ lower specification limit.

As a Tool: 6σ is a set of existing tools used for business performance improvements, to design new processes & products, improve existing processes & product.

As a Methodology: 6σ comprises of methodologies like DMAIC, DMADV, DFSS etc. The most common methodology used for improving existing processes/ products is DMAIC, which stands for five phases of a 6σ project, Define, Measure, Analyse, Improve & Control.

As a Measure: We use Sigma Level as a measure to state the capability of the process. As soon the sigma level increases (which means that more of the process spread squeezes between specification limits) the probability of getting defects decreases. A 6σ process has a probability of getting only 3.4 defects per million opportunities.

As a Metric: There are various metrics which are used in a six sigma project and serve various purposes. Ex: Defects per unit (DPU), Defects per million opportunities (DPMO), Rolled Throughput Yield (RTY), First Time Yield (FTY) etc.

As a Benchmark: 6σ level companies are world-class benchmarks. Their processes are so effective and efficient that the cost of poor quality and probability of defects is very low.

As a Goal: An organisation can make a Goal to attain the 6σ level. Though reaching to this high level of sigma is considered very challenging than to attain 2 sigma level, where simply doing standardisation can help. As sigma level goal increases, the complexity increases and more sophisticated tools are required to attain such a high capability.

As a Philosophy: Six Sigma, if followed religiously, brings a cultural change. Like it has changed the DNA of GE — “it is now the way we work — in everything we do and in every product we design.”

In whatever form you understand Six sigma, adopting six sigma brings a cultural change across the organization, with use of some simple and some sophisticated tools, in a systematic and data-driven approach to bring out the continuous improvements.

Join our  Lean Six Sigma Training  and explore this fascinating word (and world) further.