The Definition & Underlying Theory of Six Sigma

Sigma stands for the standard deviation in quality measurements.

Sigma is the symbol in probability used to represent standard deviation. Six Sigma represents six standard deviations from average of a data set. Six Sigma is also a process improvement methodology that seeks to bring a process or product to Six Sigma quality standards.

Six Sigma in Probability

If data samples are taken over a long period of time or for a large data set, roughly 68% of the data will fall within the first standard deviation markers above and below the average. At three sigma, 99.73% of the data will be included. At six sigma, 99.9997% of the data will land within the six standard deviations range.

Six Sigma in Process Improvement

Six Sigma as a process improvement methodology seeks to improve a process until it has 3.4 or fewer defects per million products. Six Sigma projects can also seek 3.4 or fewer defects per million opportunities.

Six Sigma Theory

Six Sigma theory is based on the idea that all processes produce outcomes with a normal distribution over time. Six Sigma theory also states that an average and sigma or standard deviation can be calculated from this process, allowing Six Sigma boundaries and process control charts to be calculated. Six Sigma project tools are then used to reduce variation in the process or reduce defect rates.

References

• "Six Sigma Statistics with Excel and Minitab"; Issa Bass; 2007
• "Rath & Strong's Six Sigma Advanced Tools Pocket Guide"; Augustine Stagliano, Rath & Strong; 2004
• "Six Sigma for Dummies"; Craig Gygi, Neil DeCarlo, Bruce Williams; 2005
• "Design for Six Sigma: A Roadmap for Product Development"; Kai Yang, Basem S. EI-Haik; 2008

Resources

• Read this Article in Spanish
• "The Six Sigma Performance Handbook"; Praveen Gupta; 2004