Some possibilities:

• Random: Select any attribute at random

• Least-Values: Choose the attribute with the smallest number of possible values

• Most-Values: Choose the attribute with the largest number of possible values

• Max-Gain: Choose the attribute that has the largest expected information gain. In other words, try to select the attribute that will result in the smallest expected size of the subtrees rooted at its children.

The ID3 algorithm uses the Max-Gain method of selecting the best attribute.