code: public double getStandardError() {
          Set<V> overlap = new HashSet<V>(baselineMetricPerDimension.keySet());
          overlap.retainAll(testMetricPerDimension.keySet());
  
          // paired or matched samples --> analyse distribution of difference scores
          SummaryStatistics differences = new SummaryStatistics();
          for (V key : overlap) {
              double diff = baselineMetricPerDimension.get(key) - testMetricPerDimension.get(key);
              differences.addValue(diff);
          }
  
          double e = differences.getStandardDeviation() / Math.sqrt(differences.getN());
          return e;
      }
docs: Implements equation (8.13) from "Elementary Statistics: A Problem Solving
  Approach 4th Edition", Andrew L. Comrey, Howard B. Lee
  
  @return the standard error as the ratio of the standard deviation divided
  by the sqrt(number of users) of the distribution of difference scores.