5.1System Development Techniques

System development proceeds through stages, from the upstream work of deciding "what to build" to the downstream work of confirming "does it work correctly." Understanding what gets fixed at each stage, together with the techniques that raise design and implementation quality (decomposition guidelines, test coverage criteria, reviews), is the foundation for practical understanding that carries into the Applied Information Technology exam.

5.1.1The development process and design layers

• Requirements definition is the stage that clarifies the business requirements of users and clients, defining what the system must achieve. The requirements fixed here become the baseline for every later stage, and the later a requirement change happens, the larger the rework cost.
• External design (basic design) is the stage that designs the screens, reports, and interfaces visible to users. Internal design (detailed design) is the stage that designs, from a programmer's viewpoint, the internal structure, module decomposition, and data structures needed to realize the external design. The distinction is one of viewpoint: external design is "what the user sees," internal design is "what the developer implements."
• Module decomposition is the design technique of splitting a program into functional units. The quality of a decomposition is evaluated on two axes: module strength (cohesion—how tightly related the processing inside one module is; higher is better) and module coupling (how strongly modules depend on each other; lower is better). "High strength, low coupling" is the guiding principle for good design.