Six Sigma is a tool from the "lean" organization productivity toolkit and particularly concerned with of output and services of an organization in terms of quality, productivity and effectiveness. It provides targets to be achieved by that organization. The concept originated with the Motorola organization. Six Sigma is allied to the zero defects (zee-dee) drive.
Six sigma as a management aid to quality is used to assess the level of quality obtaining in an organization or to set targets for a required quality level. The actions needed to attain these levels is the hard bit, which requires the use of various management tools available for the purpose.
Why "sigma"?
Those who have met the Greek letter "sigma" in terms of statistical analyses may be inclined to switch off at this stage, but the concept of Six Sigma has little to worry anyone on that score. In fact, it is an extremely simple concept to comprehend. Almost the only connection with statistical method is the level of defect parts or actions that must be attained to satisfy the Six Sigma requirement. So, be not dismayed but read on.
Those readers who are familiar with statistics will know that the Greek letter sigma (σ) is the symbol for the standard deviation of a sample of data. In the present context it refers to a level of services and output to be attained by the organization.
Parameters are selected on which to base the measures which are most important to the organization (usually in percentages), such as level of defects found in products completed, customer complaints, errors made in invoices and similar parameters often allied to quality of service and manufacture as, for example, in quality control. It is very much customer oriented.
For example, suppose a company dispatches one thousand orders to customers over a period and out of these there are six that contain errors (e.g. incorrect contents). What level of sigma does this signify? The answer is found in the tables of probability published in most textbooks on the subject of statistical method (although few publish these beyond 3 or 4 sigma levels). For the non-mathematical reader these are translated into plain English in the table given below. In the present example (equivalent to 6,000 defective orders out of a million dispatched) this equates to the sigma level 4, as stated in the table. To achieve Six Sigma quality there should be fewer than 4 defectives per million. As one will appreciate, this is an extremely high target to achieve and needs a great deal of improvement in the company's procedures and operation.
The six-sigma concept requires active cooperation and, importantly, action from employees and management alike. These people can be organized into teams headed by a trained and experienced Six Sigma team leader or "black belt". Six Sigma has adopted competence levels of the martial arts to indicate the competences, levels of knowledge and training of team members. One important concept is not to accept one's competence level but to strive to improve on it.
Jack Welch former chairman of General Electric is quoted (1997) as saying that "Six Sigma changes how management operates" and "Six Sigma is about putting into practice the notions of working smarter, not harder" (ref: 4 below). These are very pertinent aims which sum up the concept extremely well.
How is it done?
Continuous improvement is key to achieving these extremely high levels of quality and effectiveness. Examples of its tools and approaches are:
- Training employees to be aware of bad practices and suggest improvements
- Training employees in what is needed to achieve the sigma levels
- Methods improvements to create more effective and efficient working methods
- Improvement of working conditions to be more compatible with the concept
- Improvements in the plant maintenance procedures, using its tools such as FMEA (failure mode and effect analysis).
Extract from standard tables of probabilities (adjusted as explained below)
| Defects per million opportunities (rounded): | ||||
| Defects: | 67,000 | 6,000 | 230 | 3 |
| Sigma level: | 3 | 4 | 5 | 6 |
Important note: The above figures do not relate directly to the values found in tables of probabilities. Due allowance has been made for possible swings of about 1.5 sigma around the given textbook values. This has been adopted as generally accepted practice.
