Six Sigma is essentially a detailed analysis into the number of flaws a specific company process or operation has. For instance, a company that makes brake hoses will use a number of manufacturing processes to produce their brake hoses. A brake hose with a defect, or a batch of brake hoses with the same defect would be disastrous; not just for the company but for the car companies buying the brake hoses, as well as the end customers who buy the cars with the faulty brake hoses.
The reason a company might use Six Sigma is to prevent such defect opportunities from arising. The Six Sigma programme therefore helps employees analyse their specific manufacturing processes and identify any potential gaps or problems, which may, in any way, have a negative customer satisfaction end result.
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Defect opportunities
The Six Sigma initiative takes three important variables into account when analysing defect opportunities:
- Every single different defect that might occur on an item/ part
- The number of places on that specific item/ part where the defects might occur
- Every single step in the production process that might result in one or more item/ part defects.
How to work out your Defect Opportunity calculation
Let’s say a company is manufacturing dice for a casino. During their defect analysis, they find that two major defects are commonly occurring; a medium size crack on one dice face and a small hole on another of the faces; so two defects in all occurring on two sides of the dice. On investigating these defects, it appears that there are two steps within the process of manufacturing the dice where these defects occur.
The above shows you that there are therefore a number of defect opportunities. To work out exactly how many using Six Sigma, you multiply the number of defects in the dice by the number of sides it affects by the number of steps in the manufacturing process where these defects occur. Therefore, in this case the multiplication would be 2 x 2 x 2, which equates to 8 defect opportunities.
Next to work out the number of defects per opportunity, you need to check what percentage of dice in the manufacturing line are being affected; say in this case it is 10 percent; then you would divide 10% by 8, which equals 0.0125 and then multiply this number by 1,000. Many companies used to use this calculation to work out what the number of defects per thousand opportunities is going to be, however, this all changed when Motorola introduced DPMO.
Defects per million opportunities (DPMO)
DPMO was introduced by Motorola engineers, who came to the conclusion that working on a defects per thousand basis wasn’t accurate enough. For them, defects per million opportunities removed more errors because the sample size was smaller and therefore enabled a much more accurate quality calculation. To calculate the DPMO, simply multiply your initial calculation by 1,000,000.
The Six Sigma Scale
Motorola had a huge impact on the development of Six Sigma, introducing a scale to make it easier to evaluate the quality of manufacturing processes; basing this scale on defect calculations. This scale is shown below:
- Six Sigma – equates to 3.4 DPMO, or 99.9997% defect-free
- Five Sigma – equates to 233 DPMO, or 99.98% defect-free
- Four Sigma – equates to 6,210 DPMO, or 99.4% defect-free
- Three Sigma – equates to 66,807 DPMO, or 93.3% defect-free
- Two Sigma – equates to 308,538 DPMO, or 69.1% defect-free
- One Sigma – equates to 691,462 DPMO, or 30.9% defect-free
In this manner, you can see that if your company is operating processes at Six Sigma, you have nearly eliminated all defects, which is very nearly perfect. However, it is very rare to operate every process at Six Sigma; most companies run at Three of Four Sigma, so in all likelihood you are also probably operating at Four Sigma or lower.
If you do reach the dizzy heights of Six Sigma, however, your company will be making considerable savings since it has been estimated in the past that eliminating such defects can save a company between 1.2 and 4.5%. You can imagine how large an amount of money this would be for a multinational company.
James writes for Sigma Pro. When not writing, he can often be found teaching others that Six Sigma is nothing to be afraid of.
