Best Splits Pick best attributes and conditions on which to partition The purity of a set S of training instances can be measured quantitatively in several ways. Notation:number of classes k,number of instances =S], fraction of instances in class i=pi. The Gini measure of purity is defined as [ Gini(S)=1-Σp2, i-l When all instances are in a single class,the Gini value is 0 It reaches its maximum (of 1-1 /k)if each class the same number of instances. Database System Concepts-6th Edition 20.16 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 20.16 ©Silberschatz, Korth and Sudarshan th Edition Best Splits Pick best attributes and conditions on which to partition The purity of a set S of training instances can be measured quantitatively in several ways. Notation: number of classes = k, number of instances = |S|, fraction of instances in class i = pi . The Gini measure of purity is defined as [ Gini (S) = 1 -  When all instances are in a single class, the Gini value is 0 It reaches its maximum (of 1 –1 /k) if each class the same number of instances. k i- 1 p 2 i