A model for measuring software modifiability is presented. In this model, the key to the modifiability of a program module is attributed to its localization property, which can be interpreted as indicating whether changes made to the module would have only a localized effect or not. The localization property of a module can be further broken down into intramodular localization and intermodular localization. Corresponding to the localization property are the dependencies among program code entities. A data-interdependency metric for measuring the dependencies among primitive statements in a module is developed. The metric is then extended to incorporate intermodular localization property to measure the modifiability of program modules. A number of software maintenance experiments were designed and conducted for validating the software modifiability metric. The details of the experimentation as well as the results are presented.