asixsigma Ltd Home Page  

Six Sigma

Business Improvement


Site Updated: Fri 29 Aug 2008 23:00 BST

Deployment Training Projects IT Solutions Links About Us
Six Sigma Info Team Downloads Contact Us

 

What is Six Sigma? Why Six Sigma? How does Six Sigma increase your profits? What is a sigma rating? Who uses Six Sigma? Technical Definition

What is a Sigma Rating?

A sigma rating is a measure of the quality of a process. A process with a six sigma rating produces very good quality output. The table below shows how sigma rating  of any given process relates to the number of defective results you would expect to see if the process were to produce a million outputs. 

 

Average Defect Occurrence Rate

s rating

defects per million opportunities
1.0 697,672
1.5 501,350
2.0 308,770
2.5 158,687
3.0 66,811
3.5 22,750
4.0 6,210
4.5 1,350
5.0 233
5.5 32
6.0 3

 

 

 

The above symbol is used to quickly show the desirable direction to progress in any table or graph.

What does Six Sigma look like?

Below are some boxes illustrating their associated defect rates of different sigma ratings (at the Motorola defined worst case shift of 1.5 standard deviations). Each box is 1,000 x 1,000 pixels in size, giving the 1 million pixel opportunities for success.

  • red pixels represent flawed, defective or bad output.

  • non-red pixels (i.e. yellow & green) represent good or non-defective output 

For each box the the likelihood of a defect is visually represented by the amount of area in box of opportunity which is coloured red! 

For best viewing you require a horizontal resolution of at least 1,024 pixels, a defect free monitor and good eyesight at the higher sigma ratings! These examples show the EXACT integral numbers in one million opportunities. 

You may notice the lower sigma rated boxes take more time to load. This due to the poorer compression. This is the cost associated with the lack of predictability in the picture!

(Note: For clarity, any red pixels which would be located on the green text have been redistributed this has a small effect on the apparent density of red dot at the lower sigma ratings.)

1.0 sigma rating (697,672/1,000,000)

Back to Top

1.5 sigma rating (501,350/1,000,000)

Back to Top

2.0 sigma rating (308,770/1,000,000)

Back to Top

2.5 sigma rating (158,687/1,000,000)

Back to Top

3.0 sigma rating (66,811/1,000,000)

Back to Top

3.5 sigma rating (22,750/1,000,000)

Back to Top

4.0 sigma rating (6,210/1,000,000)

Back to Top

4.5 sigma rating (1,350/1,000,000)

Back to Top

5.0 sigma rating (233/1,000,000)

Back to Top

5.5 sigma rating (32/1,000,000)

Back to Top

6.0 sigma rating (3.4/1,000,000)

Back to Top

What is a typical sigma rating?

Typically a good business processes before application of Six Sigma techniques can be measured to perform between 3.5 and 4.5 sigma.

Do I have to produce a million process outputs before I can prove I have an "n" sigma rated process?

No using mathematical techniques a trained Six Sigma practitioner can determine using small sample quantities the sigma rating of a process' output.

We were once asked to calculate the sigma rating of a process where the sponsor was more "keen to be seen to be doing Six Sigma" than really practicing it. Their process was producing 162 defects in 267 opportunities and they wanted to know their sigma rating so it could be publicized!

If your process is producing more than 66,811 dpm (defects per million) (or 6.7% defects) for your customers then wanting to calculate your sigma rating, is rather like trying to measure an elephant with a micrometer! 

Trust us, you don't need to know your sigma rating, you need to fix your process, quickly!

(The Sigma Ratings of 1 to 3 are shown here purely for illustration!)

Do I have produce a million process outputs before I can benefit from a higher sigma rating?

Although these performance ratings are determined as a number of defects in a million opportunities, it does not mean our process needs to produce a million or more items in order to get the benefits.

Our process need only produce as little as one single output. When producing any output, the higher our sigma rating, the lower the likelihood that we will produce a defect. There is increased predictability and confidence in the process to delivering defect free output. This benefit is gained each time the process is run.



asixsigma is the trading name of asixsigma Limited which is registered in England No. 5169920

Site contents (c) 2004, 2005, 2006 asixsigma