There are various sigma levels of the processes. A sigma level of the process determines the accuracy and quality of the process. By knowing the sigma level of the process we can make out how the process is performing for eg. How many defects are occurring during the process. Higher the sigma number of the process, higher will be the accuracy and quality of it.

In Statistics, Sigma represents the Standard Deviation which measures how much variation a process shows from the perfection. The calculation is based on the number of the defects occurring per million of opportunities.

DPMO is Defects per million opportunities

DPMO = (Number of defects)*1000000/(Number of units)/(Number of opportunities for defects per unit

The relationship between the sigma level and DPMO clearly suggests the efficiency of the process. Higher sigma level means the lower value for the DPMO and increased levels of process efficiency. Vice Versa lower sigma level means higher value for the DPMO and decreased levels of process efficiency. Therefore a process should aims to achieve the higher Sigma Levels.

Relationship between Sigma Levels and DPMO is as follows

- One sigma = 690,000 DPMO = 31% efficiency
- Two sigma = 308,000 DPMO = 69.2% efficiency
- Three sigma = 66,800 DPMO = 93.32% efficiency
- Four sigma = 6,210 DPMO = 99.379% efficiency
- Five sigma = 230 DPMO = 99.977% efficiency
- Six sigma = 3.4 DPMO = 99.9997% efficiency

The performance of the processes can be compared throughout an entire organization by determining the sigma levels of the processes. Determining the sigma levels is independent of the process, as it calculates only opportunities and defects. Different processes may differ in number of opportunities for making mistakes and different number of units produced, however by comparing the DPMO of these processes we can make out which process is performing better.