Rule Definition
Changing a class requires prior understanding, which, in turn, is more complicated for classes with many methods. Classes that are deep down in the classes hierarchy potentially inherit many methods from super-classes. Moreover, the definitions of inherited methods are not local to the class making it even harder to understand it. Complete testing requires coverage of all execution paths. The number of possible execution paths of a class increases with the number of methods and their control flow complexity. Classes that are deep down in the classes hierarchy potentially inherit many methods from super-classes. Due to late binding, super-class methods need to be tested again in the sub-class context. This makes it it potentially harder to test classes deep down in the classes hierarchy. Technical or framework classes which are evolving and will not be changed often should not be concerned by this rule.

A Metrics Suite for Object Oriented Design Chidamber, S.R. and C.F. Kemerer IEEE Transactions on Software Engineering, 1994