In this publication, the model used by SIG for rating Maintainability as defined by ISO/IEC 9126 is introduced and explained.

 
A Practical Model for Measuring Maintainability
Ilja Heitlager, Tobias Kuipers, and Joost Visser, A Practical Model for Measuring Maintainability, In proceedings of the 6th International Conference on the Quality of Information and Communications Technology (QUATIC 2007), pages 30-39, IEEE Computer Society Press, 2007.

The amount of effort needed to maintain a software system is related to the technical quality of the source code of that system. The ISO 9126 model for software product quality recognizes maintainability as one of the 6 main characteristics of software product quality, with adaptability, changeability, stability, and testability as subcharacteristics of maintainability.

Remarkably, ISO 9126 does not provide a consensual set of measures for estimating maintainability on the basis of a system’s source code. On the other hand, the Maintainability Index has been proposed to calculate a single number that expresses the maintainability of a system.

In this paper, we discuss several problems with the MI, and we identify a number of requirements to be fulfilled by a maintainability model to be usable in practice. We sketch a new maintainability model that alleviates most of these problems, and we discuss our experiences with using such as system for IT management consultancy activities.

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