Is there a cliff-edge in object-oriented inheritance?

One open research question in the object-oriented (OO) community is whether a cliff-edge exists in inheritance. In other words, beyond a certain level of OO inheritance depth, are there significant differences between the composition and characteristics of classes at the two levels. Anecdotally, level three has been proposed as that psychological cliff-edge, but no empirical evidence exists to support or refute that theory. In this paper, we analyzed data from nine open-source systems to determine whether significant differences between classes at level three and those at level four existed. We looked at the number of methods, extent of coupling and cohesion in the classes at depth three and compared that with the number of methods at depth four. Results suggested no significant differences in any of the three measures. The data thus refutes the myth that there might be a “cliff-edge” level of inheritance.