The term Sigma, taken from the Greek alphabet, is used to designate the distribution about the mean of any parameter of a product, process, or procedure. In common terms, Sigma represents the capability of anything (product/procedure or process) to show how accurately it is working.
The level of Sigma corresponds to the defect rate as follows:
- 3 Sigma: 99.73% quality, 66,807 defective parts per million
- 4 Sigma: 99.9937% quality, 6,210 defective parts per million
- 5 Sigma: 99.999943% quality, 233 defective parts per million
- 6 Sigma: 99.9999998% quality, 3.4 defective parts per million
In essence, when someone claims they have achieved six sigma in manufacturing bearings or delivering food to customers, they are assuring that the chance of a defective bearing or the likelihood of wrong food being delivered to a customer is only 4 cases in 1,000,000 deliveries.
To understand "zero defect," it represents an ideal condition in which the quality percentage is 100%. In other words, there would not be a single defective bearing found over the entire plant's operation.