When we talk about Sigma Level in terms of metrics, if we know the Sigma Level, we can get an idea of how many defects the process is making. In order to compare different processes that may have different number of units of products or services and different opportunities for making mistakes, we usually talk in terms of DPMO (defects per million opportunities). The following table shows the relationship between Sigma Level and DPMO.

For example, if a process is operating at 4 Sigma Level, then if the process has an opportunity for making 1 million defects, this process will only make about 6,210 defects.

The above table only provides DPMO for only certain combinations of Sigma Levels. If you are interested in translating any Sigma Level to DPMO or vice-versa, you can use the attached Excel Spreadsheet.

Let's apply this to a real world example:

Every day there are roughly 144,000 flights that take off throughout the world. In a year, this amounts to 52.56 million flights. On an average, there are 36 plane accidents every year.

The DPMO is given as equal to ratio of the total number of defects divided by the number of opportunities for making defects multiplied by 1 million.

This corresponds to a DPMO = 0.685 (36/52.56). Here we are assuming that for each flight the opportunity for error is 1.

Using the sigma calculator, we get a Sigma Level of 6.3.

Hi Suresh,

I read once that aviation Industry operates at 12 sigma. what does it mean????

 

Mohit

Dear Mohit,

The Sigma level can be computed based on the number of defects per million opportunities (DPMO).

DPMO = (Number of defects)*1000000/(Number of units)/(Number of opportunities for defects per unit)

The number of defects and number of units can be uniquely determined. However, there is no unique way to determine the number of opportunities for defects per unit (OFE). As a result, there is no unique number for DPMO and hence Sigma Level.

This may sound counter intuitive that Sigma Level can be any value that the user wants it to be - by manipulating the OFE.

The main objective of the Six Sigma project should be to capture the baseline Sigma level before the project and then use the same definition of OFE when calculating the Sigma level after the completion of the project to show an improvement.

Let's look at the airline example,

I had used a value of OFE = 1, which is each flight can have one opportunity for error (crash). This gave a Sigma level of 6.3.

However, we could also have chosen the number of miles travelled by the plane as OFE. If on an average, a plane travels 1000 miles per trip, then we would get:

DPMO = 36/(52.6*1000) = 6.8 x 10^(-4)

This corresponds to Sigma Level of 7.6 (including the 1.5 Sigma Shift).

Of course, if someone assumes that the opportunity for error is defined as every second of plane travel there is an opportunity of a plane crash, then we would get an even higher Sigma level!

Hope this helps,

SJ>