For example, if we have 43 instances of the training set in the node of which 13 belong to one class, while 30 instances belong to a second class. Given that we have only those two classes in the training dataset, we calculate Gini impurity 1 – (13/43)2 – (30/43)2 1 – 0.09 – 0.49 0.42. When the node is "pure" its Gini index is 0. On the other hand, information gain lets us find the best threshold which will reduce this impurity the most. To calculate information gain we need to calculate average impurity and then subtract that from the starting impurity. That is how we know the quality of thresholds that we used.