Three times the standard deviation, a statistical quantity. This deviation is applied to a system's mechanical measurements, which require statistical analysis of the data to determine the system's expected performance. If the data set is normal and randomly distributed, +/- 3 standard deviations from the mean will capture 99. 74% of the data. Statistically, 0. 26% lies outside of the +/- 3-sigma limits (2. 6 points out of a thousand).