code: public Vec[] getColumnMeanVariance()
      {
          final int d = getNumNumericalVars();
          Vec[] vecs = new Vec[] 
          {
              new DenseVector(d),
              new DenseVector(d)
          };
          
          Vec means = vecs[0];
          Vec stdDevs = vecs[1];
          
          MatrixStatistics.meanVector(means, this);
          MatrixStatistics.covarianceDiag(means, stdDevs, this);
          
          return vecs;
      }
docs: Computes the weighted mean and variance for each column of feature
  values. This has less overhead than
  {@link #getOnlineColumnStats(boolean) } but returns less information.
  
  @return an array of the vectors containing the mean and variance for
  each column.