What Sigma level of quality corresponds to a 99% quality rate?

A Sigma level of quality provides a numerical representation of the amount of variation in a process, quantified by defects per million opportunities (DPMO). A 99% quality rate implies that 99% of the products or processes fall within the desired specifications, while 1% could potentially be defective.

To relate the quality rate to Sigma levels, a 3-sigma level typically corresponds to a quality rate of around 93.3% and a defect rate of approximately 66,807 defects per million opportunities. A process at a 4-sigma level indicates a quality rate of about 99.38%, suggesting that processes can be slightly improved to reduce defects further. At the 5-sigma level, the quality rate is around 99.9997%, which represents a very high quality standard with extremely few defects.

Although a direct match for 99% isn't commonly represented in terms of Sigma levels, 3.8 Sigma is generally used to indicate a quality level that closely aligns with a 99% quality rate. A quality rate of 99% does not equate to 6 Sigma, which signifies a defect rate much lower than 1 in a million.

Therefore, the relationship of 99% quality to a Sigma