![]() ![]() Control charts are used to maintain 6 sigma quality by signaling when quality professionals should investigate a process to find and eliminate special-cause variation. This is why Six Sigma aims to have processes where the mean is at least 6σ away from the nearest specification limit.Ī control chart depicting a process that experienced a 1.5 sigma drift in the process mean toward the upper specification limit starting at midnight. Even if the mean were to move right or left by 1.5σ at some point in the future (1.5 sigma shift, colored red and blue), there is still a good safety cushion. Because of the properties of the normal distribution, values lying that far away from the mean are extremely unlikely: approximately 1 in a billion too low, and the same too high. The upper and lower specification limits (marked USL and LSL) are at a distance of 6σ from the mean. For the green curve shown above, μ = 0 and σ = 1. The greater the standard deviation, the greater is the spread of values encountered. In the center at 0, the Greek letter μ (mu) marks the mean, with the horizontal axis showing distance from the mean, marked in standard deviations and given the letter σ (sigma). Graph of the normal distribution, which underlies the statistical assumptions of the Six Sigma model. ![]() So, processes, where the mean is minimum 6σ away from the closest specification limit, are aimed at Six Sigma. The higher the standard deviation, the higher is the spread of values encountered. The graph of the normal distribution below underscores the statistical assumptions of the Six Sigma model. If the process has six Sigmas, three above and three below the mean, the defect rate is classified as “extremely low.” “ Six Sigma” comes from the bell curve used in statistics, where one Sigma symbolizes a single standard deviation from the mean. The etymology is based on the Greek symbol “sigma” or “σ,” a statistical term for measuring process deviation from the process mean or target. A six-sigma process is one in which 99.99966% of all opportunities to produce some feature of a part are statistically expected to be free of defects. Jack Welch made it central to his business strategy at General Electric in 1995. It was introduced by American engineer Bill Smith while working at Motorola in 1986. Six Sigma (6σ) is a set of techniques and tools for process improvement. ![]()
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