This article proposes metrics for quantifying changes throughout the evolution of formal software specifications in long living systems. Formal specifications play an important role in the software development life-cycle by supporting refinement and proof and by providing a basis for comprehension and maintenance activities. However, specifications also undergo evolutionary changes, and these changes are hard to assess because of a lack of suitable measures.
This paper proposes and analyzes a set of measures for estimating aspects of a specification's complexity and quality. The measures are based on existing measures for source code, but they have been redefined in the scope of formal Z specifications. Geared towards Z, they are then evaluated concerning their expressiveness by a case study that comprises more than 65,000 lines of specification text. Finally, the usability of the measures is demonstrated on the Z specification of the Web Service Definition Language during its evolution over a period of about 3 years. Copyright © 2013 John Wiley & Sons, Ltd.