Surface roughness can be measured as either an arithmetic average (aa) or a root-mean-square (rms) average. which value is almost always greater for most surfaces:

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.

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