Decision Tree, Entropy, Information Gain, Gini Index.
there is a a mixture of Pass and Fail in a sub node, there is scope to split further to try and get it to be only one category. This is termed the purity of the node. For example, Not Working has 5 Pass and 1 Fail, hence it is purer than the Working node which has 5P and 4F. A leaf node would be