The core of the Six Sigma quality management approach is measurement of defects in order to get as close to zero defects as possible. Sigma is a statistical term that measures how far a process deviates from total accuracy or perfection. The process sigma, which is also known as the sigma level, is a measure of process capability. The higher the process sigma, the more capable the process is. A Six Sigma process has a short-term (DPMO) process sigma of 6. When determining the long-term process sigma, 1.5 is subtracted from the short-term metric, so that the long-term process sigma for a Six Sigma process is 4.5.
Six Sigma is often wrongly defined as "3.4 defects per million products," when in fact, Six Sigma is actually defined as 3.4 defects per million opportunities (DPMO). Six Sigma's goal is to improve all processes to that level or better.
To determine the number of opportunities a process contains, one should think of the number of opportunities in which a defect may occur. For example, if you are measuring emergency department (ED) stat turnaround times from order to completion, a defect would be any result not reported within the specified turnaround time. Opportunities for defects (delays) can occur in the three phases of laboratory testing (preanalytical, analytical, and post-analytical phases).
An example of DPMO and process sigma (sigma level) measurement is given on the following page.