code: public static double meanDoublematrixColumn( double[][] matrix, int column ) {
  
          double mean;
  
          mean = 0;
  
          int length = matrix.length;
          for( int i = 0; i < length; i++ ) {
              mean += matrix[i][column];
          }
  
          return mean / length;
  
      }
docs: Return the mean of a column of a matrix.
  
  @param matrix matrix of the value to calculate.
  @param column index of the column to calculate the variance.
  @return mean.