In its most fundamental form, Six Sigma is a measure of the number of defects in a specific process or operation -- for example, a manufacturing process used to make a specific part. In Six Sigma, you're not worried about defective parts as a whole, but something called defect opportunities. A defect opportunity takes into account three important variables:
- All of the different defects that occur on an assembled part
- The number of places on that part where the defects can occur
- And every production step that could cause one or more of the defects on the part
As an example, let's say you're manufacturing small metal cubes. Two major defects are typically found on the cubes: a crack and a dent. The crack is one defect; the dent is a second. Now let's say those defects are found only on three of the cube's six faces. Finally, let's assume there are three steps in the manufacturing process where those defects are typically introduced.
Clearly, there are several opportunities for a defect to occur. To calculate how many, you simply multiply: 2 x 3 x 3, for a total of 18 opportunities. Now, if you see cracks or dents in 5 percent of the metal cubes that come off the production line, the number of defects per opportunity is .00278 (.05 divided by 18). To find the number of defects per thousand opportunities, you multiply .00278 by 1,000 to get 2.78.
Motorola engineers decided that the defects-per-thousand metric wasn't sensitive enough for their new Six Sigma initiative. They decided that defects per million opportunities (DPMO) eliminated errors due to small sample size and made for a more accurate determination of quality. To find the number of defects per million opportunities in our example above, you multiply .00278 by 1,000,000 to get 2,780 DPMO.
On the next page, we'll discuss the scale that Motorola came up with to evaluate quality based on DPMO numbers.