When the selection of tests is interleaved with classification in this way, a decision tree is
the natural form of representation. The root of the decision tree represents the first test that
we choose. The next level of the decision tree represents the next test that we choose. The
decision tree explicitly shows how the outcome of the first test determines the choice of the
second test. A leaf represents the point at which we decide to stop testing and make a classi-
fication.