In Six Sigma, Defects per Million Opportunities (DPMO) is the measure of the process performance.
Defect vs Defective:
Let's first understand the difference between the defect and defectives.
Defect:
A defect is the non-conformance on one of many possible quality characteristics of a unit that causes customer dissatisfaction.
ISO 9001:2015 defines a defect as the nonconformity related to an intended or specified use.
Defective:
A unit which has one or more defects is a defective unit.
Example: In the image shown below, each mistake on the page is a defect. However, the whole unit (the page) is the defective unit. Here in this example, we have one defective which has multiple defects. The term DPMO uses the count of defects and not the defectives.
Defects per Unit:
Defects per Unit (DPU) is a measure of the average number of defects per unit. Let's assume that a page is a unit in the example we used above. When we checked 10 pages (or units), we found a total of 2 defects in those 10 pages.
One defect was related to the spelling mistake and another related to the grammar.
Hence DPU = 2/10 = 0.2
Defect Opportunity:
Defect Opportunities are the circumstances in which the Critical to Quality parameter fails to meet the customer requirement. Remember that defect opportunity is the "potential defect" and not the actual defect.
The number of defect opportunities in a unit (product, process or service) is related to the complexity of the unit. More sophisticated units will have more opportunities of defects as compared to simpler units.
Examples: A unit has 5 parts, and in each part, there are 3 opportunities of defects – Total defect opportunities are 5 x 3 = 15
Defect per Opportunity (DPO)
Defects per opportunity (DPO) is the number of defects identified in a sample divided by the total number of defect opportunities.
Examples: In the previous example we identified 15 defect opportunities per unit. Let's assume that on inspecting 10 such units we found out 2 defects in those.
DPO = 2 / (15 x 10) = 0.0133333
Defect per Million Opportunities (DPMO)
Defects per Million Opportunities is calculated by multiplying DPO by one million.
Examples: Continuing with the previous example
DPMO = 0.013333333 x 1,000,000 = 13,333
Sigma Level
There is a relationship between the DPMO and the Sigma Level of the process. Six Sigma performance is equivalent to 3.4 DPMO. Click here for the detailed calculation.
13,333 DPMO found in the example shown here is equivalent to 3.7 Sigmas.
Sigma Level (with 1.5 sigma long term shift) | Defects per Million Opportunities (DPMO) | Percentage Yield |
---|---|---|
1.5 Sigma | 500,000 | 50% |
2 Sigma | 308,538 | 69% |
3 Sigma | 66,807 | 99.3% |
4 Sigma | 6,210 | 99.38% |
5 Sigma | 233 | 99.977% |
6 Sigma | 3.4 | 99.99966% |