code: public static double mean(TransposeDataCollection sampleDataCollection) {
          double mean = 0.0;
          
          int totalSampleN = 0;
          for(Map.Entry<Object, FlatDataCollection> entry : sampleDataCollection.entrySet()) {
              mean += Descriptives.sum(entry.getValue());
              
              totalSampleN += entry.getValue().size();
          }
          
          mean/=totalSampleN;
          
          return mean;
      }
docs: Calculate the mean from the sample
  
  @param sampleDataCollection
  @return