Question

Is Six sigma actually realized at 3 standard deviations below the mean? Or is it six and no one is ever really achieving it?

Answer 1

In any manufacturing process, the product should defect free but there is a small percentage of defects may be existing. Likewise, in any normal distribution, the data should be distributed relatively uniform to fit in the curve. Thus, the standard deviation is the data spread to represent the wider aspect of the normal curve. If we apply the same in real life, the normal distribution focuses into two aspects, i.e. average or mean and standard deviation. And 34.1% of data appears in 1 standard deviation (above or below), 13.6% of data appears in 2 standard deviation (above or below) and 2.1% of data appears in 3 standard deviation (above or below). Overall, the size of the standard deviation changes with respect to the data and likewise the curve appears wide or flat based on the data. In case the curve is narrower or taller, the data is closer to the average or mean. Whereas, the data scattered then the curve becomes shorter and wider. Therefore, in six sigma methodology, the goal is always to restrict the product defects to “six” sigma (i.e. six standards deviations). Here, the 3 standard deviations stand (above the average or mean) for Upper specification limit whereas Lower specification limit for below the average. And these signify the limits of acceptable defect rates as termed by six sigma methodology. Overall, percentage values among the 3 standard deviation comes from the mean of 99.73% to represent the 3.4 defects per million products among which 0.00034% are defective.