Large software systems are often characterized by a continuing evolution, where a large number of people are involved in maintaining and extending the system. Software modifiability is a critical issue in such system evolution. It is desirable that the basic design is modifiable, and that subsequent evolution maintains this modifiability. This thesis is an investigation of the mechanisms behind the exhibited modifiability and lack of modifiability in a large commercial software system during a part of its evolution.

First, the relation between modifiability and different types of modularizations are discussed, and a dichotomy of software modularizations Is proposed. As a measure of modifiability at system level, i.e. disregarding the internal modifiability of modules, we use the number of modules which are influenced by the implementation of a certain system change. The implementation of each requirement in one release of the system is examined, and the underlying causes of good and bad modifiability are explained. This results in a list of factors which were found to Influence the modifiability.