code: public static double nBar(TransposeDataList clusterIdList) {
          int populationM = clusterIdList.size();
          
          double nBar = 0.0;
          for(Map.Entry<Object, FlatDataList> entry : clusterIdList.entrySet()) {
              nBar += (double)entry.getValue().size()/populationM;
          }
  
          return nBar;
      }
docs: Returns the mean cluster size.
  
  @param clusterIdList
  @return