Modeling Software Defect Introduction

In software estimation, it is important to recognize the strong relationships between Cost, Schedule and Quality. They form three sides of the same triangle. Beyond a certain point (the “Quality is Free” point), it is difficult to increase the quality without increasing either the cost or schedule or both for the software under development. Similarly, development schedule cannot be drastically compressed without hampering the quality of the software product and/or increasing the cost of development. Software estimation models can play an important role in facilitating the balance of the three factors. This paper presents an initial version of the Defect Introduction sub-model of the empirical quality modeling extension to the existing COCOMO II software cost estimation model. The Quality model is an estimation model that can be used for predicting number of residual defects/KSLOC (thousands of Source Lines of Code) or defects/FP (Function Point) in a software product. It can be applied in the early activities such as analysis and design as well as in the later stages for refining the estimate when more information is available. It enables ‘what-if’ analyses that demonstrate the impact of various defect removal techniques and the effects of personnel, project, product and platform characteristics on software quality. It also provides insights on determining ship time, assessment of payoffs for quality investments and understanding of interactions amongst quality strategies. The Quality model has two sub-models, namely the Defect Introduction model and the Defect Removal model. This paper focuses on the Initial version of the Defect Introduction model. It gives a detailed explanation of the rationale behind the suggested numeric ratings associated with the model parameters.