rms
The rms average amplifies the effect of isolated large deviations from the mean, while the arithmetic average evens out the larger deviations. For instance, let's assume you have a nice surface finish that is about +/- 3 microns from the desired surface and have one large defect of +10 microns. Let's see what happens with the math:
Surface roughness measurements:
-2.49, 2.5, -1.67, -1.62, -1.72, 0.4, -1.75, -1.63, 2.07, 10
The AA value for the 1st 9 values in the above list is 1.761111
The RMS value for the 1st 9 values is 1.855505
Notice that they're both fairly close to each other.
Now let's recalculate using all 10 values.
AA value is 2.585. A jump of 0.8238.
RMS value is 3.6192. A jump of 1.7637. The increase in the RMS value is over twice that of the AA value.
