Last spring the ISO 25010 standard for software product quality was published by the International Organization for Standardization (ISO) as a successor of the ISO 9126 standard that has been in use since 2001. The quality model of the Software Improvement Group, has been updated to reflect the change from ISO 9126 to ISO 25010. What important changes have been made?

   

The quality model of SIG maps measurements of software properties such as complexity and duplication to ratings on a scale from 1 to 5 stars. The model is refreshed yearly to reflect the state of the art in software development. In past years, the changes were limited to small adjustments of the thresholds that are used to distinguish higher and lower levels of quality.

     
But this year, some additional changes are made to the quality model, related to the publication of a new version of the ISO standard for software product quality. For several years now, the ISO has been working to bring various related standards together into the new 250xx series of standards. The name of this series is SQuaRE: Software Quality Requirements and Evaluation. The ISO 9126 standard on software product quality has been brought into this series under the number ISO 25010, published this spring.

Over the past months, the research team of SIG has worked to analyze the changes between old and new standard, and to translate them to changes in the SIG quality model.

The most relevant changes are the following:

Changes in the definition of maintainability

• The ISO definition of "maintainability" has been changed by extending it with two characteristics: "reusability" and "modularity".
• Also the separate characteristics of "changeability" and "stability" have been merged into a single characteristic called "modifiability".

Changes in the measurement areas

• The measurement areas of the SIG model have been extended with two new metrics: "component balance" and "component independence". These metrics quantify how well the system has been divided into top-level components.

Changes in the mapping from measurement areas to quality characteristics

• The two new metrics for balance and independence of top-level components are mapped to a rating for the new "modularization" characteristic.
• Additionally "component balance" is mapped to the existing "analyzability" characteristic and "component independence" has been mapped to "modifiability". Empirical research by SIG and Technical University Delft has shown statistical correlations that justify these mappings (see [1] and [2]).
• Since reuse happens by first creating right-sized reusable units and then interfacing with these units from various use contexts, the metrics for "unit size" and "unit interfacing" have been mapped to the new "reusability" characteristic.

 
In addition to these changes driven by the replacement of ISO 9126 by ISO 25010, the thresholds of the quality model have of course been re-calibrated to match the latest development in the state of the art of software development.

In spite of these changes, the SIG quality model remains the same in many aspects. It remains based on the latest ISO standards. It remains the reference for technical quality of software products. And it remains a reliable early warning system for software-related risks.

References
[1] Eric Bouwers, Arie van Deursen, and Joost Visser, Dependency Profiles for Software Architecture Evaluations, in proceedings of the 27th IEEE International Conference on Software Maintenance (ICSM 2011), p. 540-543, IEEE, 2011.

[2] Eric Bouwers, José Pedro Correia, Arie van Deursen, and Joost Visser, Quantifying the Analyzability of Software Architectures, in proceedings of the 9th Working IEEE/IFIP Conference on Software Architecture (WICSA 2011), p. 83-92, IEEE Computer Society, 2011.

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