Design of Software Development Architecture Comparison of Waterfall and Agile Using Reliability Growth Model

This paper proposes a method for deciding whether to insert an agile process as part of a waterfall project. Recently, many software projects adopt an agile software methodology. Still, some software is developed with traditional waterfall methodologies. Agile methods claim a strength of flexibility for uncertain changes, yet in some cases the initial expected scope of the project cannot be realized or undetected errors remain because schedules are fixed and unexpected backlog of tests and bug fixes remain unaddressed. On the other hand, a waterfall methodology can include high risk of violating schedule targets, while fulfilling the initially expected scope with comprehensive tests so that more complex products are reliable. For the decision whether to develop in waterfall or agile, our approach is to evaluate the effects on uncertainties by adoption of agile techniques. We begin with focus on uncertain rework. The effects on rework are evaluated as cost using simulation. The decision making problem is modeled as a decision tree. In the simulation, a Software Reliability Growth Model is used as an error likelihood and detection model. This proposed method is demonstrated using a simple shopping web site. As a case study, the effects on rework by adoption of agile can be evaluated using the developed simulator. With comparison of predicted rework costs given a balance of waterfall or agile methods for a specific case, the project can be designed more effectively.