What is Six Sigma?
Six Sigma is a long term, forward thinking initiative designed to fundamentally improve the way of doing things so that quality improves and variations are reduced.
Six Sigma is more sophisticated than applying simple problem solving tools. Six Sigma applies statistical tools to identify and remove causes of variation.
A process performing at a Six Sigma level means that 99.99966% of the products produced are statistically expected to be within specifications.
Statistical Meaning of Six Sigma
Lets consider a process which has upper specification limit (USL) and lower specification limit (LSL). Lets say a pipe with diameter 3cm has to be produced and the USL and LSL are 3.1cm and 2.9cm respectively. Each pipe that will be produced will never be exactly 3.0 cm. The measurements are likely to be 3.01cm, 2.99cm, 2.92cm, 3.09cm and so on. The deviation from the mean measurement is called the standard deviation or SIGMA. For e.g. the mean could be 3.001cm.
Six Sigma defines that the SIGMA should be so low that you should be able to fit 6Sigmas on either side of mean. Higher the variation from mean, the higher the chances that your sigma will be higher and higher the chance that you wont be able to fit 6 sigmas on either side of mean.
The diagram above represents a process where the measurements are such that 6 sigmas are fitted between USL and LSL. This represents 0.002 per million defects or 2 per billion cases.
In the real world, the mean shifts by about 1.5 SIGMA on one side of mean, thus you will be able to fit 4.5 sigmas on one side of mean and 7.5 sigmas on the other. Thus, you can have 3.34 defects/million on one side and zero defects on the other side. If you achieve this, you would have achieved six sigma accuracy.
Mumbai Dabbawala – A case study of Six Sigma
A dabbawala is a person in Mumbai, who is part of a delivery system that collects hot food in lunch boxes from the residences of workers in the late morning, delivers the lunches to the workplace, predominantly using bicycles and the railway trains, and returns the empty boxes to the worker’s residence that afternoon.
It is estimated as per various studies and surveys that dabbawalas make less than one mistake in every six million deliveries. That is better than 3.34 defects in 1 million as required by Six Sigma process. Therefore we can say that Dabbawalla network operates at Six Sigma efficiency.