What are Natural Tolerances?

Although many distributions have tails that theoretically extend to plus or minus infinity, the spread of the distribution is often described by the Natural Tolerance. The Natural Tolerance of a process or population is the range of the middle 99.73% of a distribution. For a Normal Distribution, this corresponds to a 6 standard deviation spread. For non-normal distributions, this range must be found by fitting the distribution and finding the middle 99.73%.