A: An overview of the mechanism used in Gauss levelling calculation can be found in the Help file under the Data Levelling section. A more detailed explanation is as follows:

  1. The data is ranked.
  2. The rank value is made into a p value using p = (Rank + 0.5)/size
  3. The inverse cumulative probability (z) is calculated using this implementation http://commons.apache.org/proper/commons-math/apidocs/org/apache/commons/math3/distribution/NormalDistribution.html .

This is done one group at a time, and each group has no effect on any other. Small groups are ignored, so the output remains null for these groups.