Last Updated on 15 September 2023
Defects per Million Opportunities is one of the key metrics of Six Sigma, in fact:
Your process has reached Six Sigma level when it reaches a DPMO of 3.4.
We first need to define the main terms used, which will be used throughout Six Sigma:
Defects per Unit (DPU)
Defects per Unit or DPU for short is the number of defects per unit you make (on average). A defect is where the variation is such that the part doesn’t meet the required specifications.
DPU = number of defects / number of units produced
Opportunity
An opportunity is anywhere where something can go wrong, e.g. if a product can be the wrong length, the wrong color or produced late, that is three opportunities. These are usually linked to your critical to quality (CTQ) criteria, so what you are measuring for is linked to what the customer wants. The formula is:
Total opportunities = Opportunities per unit x number of units produced
A defect is something that has gone wrong, so if an individual defective product is the wrong length and the wrong color (and both are defects that the customer would reject the part for), you have two defects, even though you only have one defective product. It is therefore possible to have more defects than defective products.
Defects Per Opportunity
The first defect measure is Defects per Opportunity, or DPO, which has a simple formula:
DPO = Defects / Opportunities for defects =DPU x opportunities per unit
As there can’t be more defects than opportunities for defects, the result will always be a number between zero and one, with zero being no defects (best case scenario), and one being every opportunity for a defect resulted in a defect (worst case scenario).
Defects Per Million Opportunities
Six Sigma requires a very low tolerance for errors, and so as our processes improve we soon can’t use DPO due to it being a very small number. The ever decreasing numbers become harder to interpret.
We therefore change to use DPMO, which is Defects Per Million Opportunities.
DPMO = DPO x 1,000,000
= 1,000,000 x Defects / (Opportunities per unit x number of units produced)
Defects vs Defective units
I should make it clear that for all of these, you are counting defects NOT defective units. If you have a defects per million opportunities of 50, and you’ve had a million opportunities, you don’t necessarily have 50 defective units. You could have one unit with 50 defects on it and everything else is perfect!
DPMO to Six Sigma
DPMO is the main way that we calculate sigma levels, without which you can’t know when you get to Six Sigma capability. You can convert between DPMO and Sigma level using lookup tables, or (my personal favorite) using Microsoft Excel.
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