Six-Sigma Quality Programs
- Publication:
- Quality Progress
- Date:
- June 1993
- Issue:
- Volume 26 Issue 6
- Pages:
- pp. 37-42
- Author(s):
- McFadden, Fred R.
- Organization(s):
- University of Colorado, Colorado Springs, CO
Abstract
Workers in a six-sigma program minimize defects and increase customer satisfaction. Components of a six-sigma program are: the improvement process, quality measurement, quality initiatives, and improvement tools. Six-sigma in a single-part or single-step process means that the chance of the process falling beyond upper and lower specification limits is at the level of six standard deviations from nominal. This is equivalent to the process having a capability index of Cp = 2. If the process mean is not centered at nominal, then it is shifted, on average, 1.5 standard deviations from nominal. This is expressed as a capability index of Cpk = 1.5, which can be interpreted as a rate of one defect per 3.4 million parts. For processes with multiple parts or steps, the defect rate can be calculated by summing the defect rates for individual components. The most important assumption about six-sigma is that process parameters are normally distributed. Therefore, managers should be aware that nonnormal distributions may lead to errors in process capability measurements.