The effects of software design complexity on defects : a study in open-source systems

Abstract

The aim of this thesis is to investigate whether there is a general correlation between post-delivery defects and system design complexity by studying measures relating to Data, Structural and Procedural complexity in object-oriented systems and determining their effect on post delivery defects. A further aim is to determine whether, during the detailed design phase, measured Data Complexity can estimate measured Procedural Complexity and Class Size for the implemented system. This research is based on prior work of Card and Glass, who introduced a System Complexity Model as a combination of Structural and Data Complexity. They applied their model to eight similar FORTRAN (RATFOR) systems. This research both investigates and extends the Card and Glass model for applying to the object-oriented environment. Several adjustments are made to accommodate important characteristics of object-oriented design and language, such as "inheritance" and "encapsulation". Based on these adjustments, a new System Complexity Model is proposed, which is then applied to 104 open-source systems to investigate its effectiveness in estimating post-delivery defects. The necessary data are extracted from the source code of systems maintained within SourceForge - a popular open-source repository. Included in the data are, Version Downloads and the Number of Developers considered as independent variables for predicting user reported defects. The Spearman's rank correlation coefficient and Generalized Linear Model (GLM) with Poisson distribution are used to analyze the collected data. The results show that the newly proposed System Complexity (Structural + Data) is not significant for estimating the volume of post-delivery defects (Post-DD). When Structural and Data Complexity are analyzed separately, the results show that Structural Complexity is highly significant in estimating the number of post-DDs. Other important findings include: 1) Data Complexity can effectively estimate Procedural Complexity and Class Size, 2) The ratio of System Complexity and Procedural Complexity is useful for estimating the probability of Defect Density and Class Size. This ratio represents the mappingofmetricsobtained during thedetailed design phase with Procedural Complexity which is measurable during implementation (writing of the source code). Show more