(See Figure 1.) Figure 1: Normal Distribution With Mean, Z-score and Six Sigma Specification Limits Therefore, a process data point can be 6 standard deviations from the mean and still be acceptable. The farther the specification limits are from the mean, the lower the chance of defects.Ī Six Sigma process has a specification limit which is 6 times its sigma (standard deviation) away from its mean. Any value beyond the specification limit indicates a defect or unacceptable result. In a process, deviations from the target or mean are accepted to a certain value defined by the specification limits (SL) around the mean. Z = (31- 25) / 2 = 3 Specification Limits and Defect Rates This count is denoted by sigma level, Z, also known as Z-score, as shown below. If the standard deviation is 2 seconds, the same point is 6/2 or 3 standard deviations away from the mean. This distance can also be measured by counting the number of standard deviations in the distance. For example, a data point with a value of x = 31 seconds is 6 seconds away from a mean value of 25 seconds. ![]() The distance from the mean μ to a data value in terms of data units can be measured. Process data usually has a normal distribution. A larger standard deviation indicates that a data set has a wider spread around its mean. Standard deviation shows the extent of variation or spread of data.
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