Defects in the Six Sigma Process

Defects in the Six Sigma Process
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  • 0:00 Defects
  • 1:31 Defects Per Unit
  • 2:13 Defects Per Opportunity
  • 4:10 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

The goal of Six Sigma is to reduce defects. But what exactly are defects, and how should they be measured? In this lesson, we will examine defects in the Six Sigma process, including how they can be defined and how to calculate different defect rates.


Tamran is the chief operating officer of a company that makes bicycles. Mostly, their bicycles are very good. But every once in a while, one of them comes off the assembly line with a major problem, like the brakes don't work or the handlebars are bent.

Tamran wants to reduce the problems in their manufacturing process in order to avoid having bicycles coming out wrong. She's heard that Six Sigma, a program focused on reducing defects, is a good way to help her straighten out her problems.

But Tamran is a little confused. She gets that the main goal of Six Sigma is to reduce the number of defects in a product. But she isn't sure what that means. She knows that brakes not working on a bicycle is a defect, but what if a bike comes off the assembly line with both broken brakes and bent handlebars? Is that one defective item, or two? Does it matter?

In Six Sigma, a defect is a failure of a product or process. Defects are a major part of the Six Sigma program because they point to a problem that needs to be solved. For example, if Tamran's bikes have bent handlebars, that's a defect. But it's also a hint that there's a problem creating that defect, and she needs to fix it. In Six Sigma, the goal is to reduce the number of defects to fewer than 3.4 per million. So Tamran should be able to make a million bikes, while only having about three of them be defective.

In order to reduce the defects in her product, though, Tamran has to first know how to measure defects. Let's take a closer look at the two ways that defects are measured in Six Sigma.

Defects Per Unit

Let's go back to Tamran's problem for a minute. A bike comes out of her company's manufacturing plant with both broken brakes and bent handlebars. Is that considered one defect or two?

The answer to that question depends on how you look at defects. The simple way to calculate defects is to calculate defects per unit, which is the number of defective items divided by the number of units inspected.

In this case, the bike that has both bad handlebars and bad brakes is only counted as one defective item. It is a 'bad bike' and is counted as one defect overall. If Tamran inspects 100 bikes and this is the only one with problems, then her defects per unit rate is 0.01: 1 in 100.

Defects Per Opportunity

Defects per unit is simple, and Tamran feels like she can calculate it easily. So that's it, right? She shouldn't even look beyond defects per unit.

Well, not so fast. You see, there's a major problem with using defects per unit as a measure. Specifically, it counts each defective unit as one without acknowledging the myriad parts that go into a unit. For example, a bike is a relatively complex piece of equipment. Many things can go wrong: the handlebars could be bent, the chain could be broken, the brakes could not work, and so on.

But think about how much more complex a car is than a bike. There are even more opportunities for things to go wrong when building a car than when building a bicycle. But defects per unit count all defective units the same. This could lead to a bike's defect rate being close, or even equal, to that of a car. If a bike's defect rate is the same as a car's, there's a big problem with that bike!

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