Atlas Copco ACTA 3000

ToolsTalk ACTA
156
99.994%
68.26%
-
σ
+
σ
-2
σ
+2
σ
95.44%
99.73%
-3
σ
+3
σ
-4
σ
+4
σ
µ
Sigma and Normal Distribution
The sigma interval tells us the percentage of all results found within the sigma
limits. The 2-sigma interval (±σ) covers 68.26% of the distribution, 4-sigma
(±2σ) covers 95,44% and so on. See the diagram above.
This means that, for example, if you perform 1000 tightening operations, 997 of them will be within the
3-sigma limits. This is valid for any normal distribution. The most frequently occurring multiple in the
industry is the 6-sigma multiple (±3σ), which is used in statistical process control and when performing
capability studies.