The Triangular Life Cycle Model

Everyone knows that the waterfall life cycle model suffers from a number of problems but in spite of this, it continues to be the most widely used life cycle model. This paper argues that many of these problems stem from project management best practices that are inappropriately applied in the waterfall model. A different life cycle approach is proposed that emphasise the product life cycle rather than the project life cycle, quality management priorities rather than project management priorities and views of quality rather than views of the project schedule. A quality management tool based on different views of quality is used to identify the “gaps” that inevitably exist between a user’s needs, the requirements specification and the product that is delivered. This is followed by a brief discussion of how these “gap” can be closed. The paper concludes by pointing out that the triangular life cycle model can peacefully coexist with project management best practices but will provide some balance to the dominant project priorities of staying on schedule and within budget. The waterfall life cycle model The waterfall life cycle model is the best known and most widely used life cycle model. A recent survey found that more than a third of organisations still base their software development life cycle on the waterfall model [Lap08]. The introduction of the waterfall life cycle model is frequently attributed to Winston Royce [Roy70]. Interestingly, “waterfall life cycle model” is never mentioned in the Royce’s article. In fact, he seems to argue for a more iterative approach to software development! So it would seem that the waterfall lifecycle model is rather like an “urban myth” [Wik08] everyone claims to know its source but it does not stand up to a close examination of the facts. One possibility source of the term “waterfall” is the typographic layout of the diagrams in Royce’s paper, which seem to suggest a waterfall. Figure 1: The original waterfall? Another possibility is that the waterfall life cycle model is more closely related to project management practices rather than software engineering. Project priorities The definitive guide to project management best practice is the PMBOK standard [PMBOK00]. PMBOK identifies 44 best practice project management processes which are organised into nine knowledge areas. The knowledge areas cover topics such as the management of human resources, communication, risk and procurement in a project context. But for most project managers the four areas that are uppermost in their minds are time, cost, scope and quality.