Validation practices should be built into every stage of the requirements process in order to ensure a quality product. Studies [36] have shown that errors introduced during requirements are the most costly to repair because of their far reaching implications into the system. Furthermore, as depicted in Figure 5, it is generally accepted that the relative cost to repair a software error progressively increases at later stages of the lifecycle model, thereby underlining the importance of early V&V activities. Typical methods include reviews, prototypes, models and acceptance tests [37].