Atlas Copco BLM 3750 - Cp Index Calculation and Meaning

BLM 3750 User Guide Appendix B – Processes and Statistics
9836 3078 01 Edition 2.0 71 (78)
as can be seen in the diagram below:
Characteristics of normal distribution
Cp index
The most commonly used functioning indices are Cp and Cpk: Cp (process efficiency) is the index of
tolerance at 6 sigma.
The formula for its calculation is:
σ ⋅
−
=
6
minmax
LL
Cp
The 6 σ in the Cp formula derives from the fact that, in normal distribution, 99.73% of the values have
dispersion of 6σ (±3σ).
We can see from this formula that the Cp values can deviate by very little (almost zero) or by a lot (very
high positive numbers).
When the Cp value is less than 1, the tolerance is less than the 6σ amplitude; when the Cp value is greater
than 1, the tolerance is greater than the 6σ of the distribution.
0,135 %
0,003 %
+4 σ
+3 σ
+2 σ
+1 σ
-1 σ
-2 σ
-3 σ
-4 σ
68,26 %
95,44 %
99,73 %
99,994 % 0,003 %
0,135 %
X