The Case Against DPMO

Measuring quality performance using DPMO (defects-per-million-opportunities) is not a common practice, for three reasons.

The Supposed Advantage of DPMO

DPMO was created to put simple and complex processes on an even playing field by “normalizing” the defect rate.

The normalization is done by dividing the total observed defects by the total opportunities for defects to occur (more on the calculation here).

Here are the three reasons why DPMO is counterproductive –

#1 Customers Want Zero Defects. Period.

Imagine that your new car fails to start, three weeks after you purchase it. You call the dealer and their response is, “we will take care of the problem, but remember that your car is a complex machine with many opportunities for things to go wrong!”

As ridiculous as this sounds, it is exactly what we do with DPMO.

Customers don’t care about product complexity. They expect zero defects.

#2 Opportunities for Defects are Anyone’s Guess

The denominator in DPMO is the total number defects that could have occurred.

Think of a lawnmower blade and the possible defects that could occur during the manufacturing process. One team might say that blade sharpness, mounting hole location, and mounting hole diameter are the possible defects – three defects in total.

Another team might decide to add blade hardness, balance, and cutting-surface angle to the list – six defects in total.

Thus two teams would have VERY different DPMO calculations for the same number of defects!

#3 DPMO Creates a False Sense of Security

As stated above, the DPMO calculation dilutes the true defect rate by dividing it by the number of defects that could have occurred.

Let’s say that a given product has a failure rate of two percent, meaning that two percent of all products failure for one reason or another.

Now imagine there are 20 possible defects that could occur within each product. The DPMO in this case would be two percent divided by twenty, multiplied by a million, or 1,000 DMPO.

The 1,000 DPMO number seems reasonable and might not result in any improvement efforts, when in fact the 2% defect rate will create a great deal of customer dissatisfaction and negative product reviews.

Conclusion – Always Keep Your Metrics Simple and Customer-Focused

Hopefully the above examples illustrate the point that using DPMO is often counter-productive. Stick with simple quality measures that customers can relate to!