In this chapter, the stages of the Design Synthesis Phase of the method, namely the Statement of User Needs Stage, the Composite Task Model Stage, and the System and User Task Model Stage, will be described in the order by which design is advanced. The account includes the design products derived to support target system specification using the method. Using the format outlined in Chapter Three, design activities of each of the stages are described in terms of sub-processes that transform its inputs into a number of products. As in Chapter Four, case-study examples are used to illustrate the products.

The Statement of User Needs (SUN) Stage

The Statement of User Needs Stage summarizes the conclusions of extant systems analysis and defines user requirements for the target system. Thus, the information collated would include a mixture of the following:

1. (a) existing user needs and problems;

2. (b) existing design requirements, rationale and constraints;

3. (c) rationale underlying extant design features to be ported to the target system;

4. (d) performance criteria and domain semantics for the target system.

The primary purpose of the products derived at this stage is to establish constraints to support later design decisions and extensions, e.g. during the synthesis of task models at the Composite Task Model Stage.

Figure 5-1 shows the location of the Statement of User Needs Stage relative to other stages of the method (the stage is indicated by a box outlined in bold).