code: def _calculate_variance_reduction(self, y, y1, y2):
      var_tot = calculate_variance(y)
      var_1 = calculate_variance(y1)
      var_2 = calculate_variance(y2)
      frac_1 = len(y1) / len(y)
      frac_2 = len(y2) / len(y)
  
      # Calculate the variance reduction
      variance_reduction = var_tot - (frac_1 * var_1 + frac_2 * var_2)
  
      return sum(variance_reduction)
  
imports: from __future__ import division, print_function
  import numpy
  from mlfromscratch.utils import divide_on_feature, train_test_split, standardize, mean_squared_error
  from mlfromscratch.utils import calculate_entropy, accuracy_score, calculate_variance
label_desc: Use image node_id 1 for calling the RegressionTree obj's underlying member method code with example usage: obj._calculate_variance_reduction(y, y1, y2) and returns: sum