2.2 AGENTS OF SYSTEMATIC DESIGN
In addition to a system model of the design, models of user interaction with the system can be expressed. Table 3 below lists and describes four evaluation modes. A constraint-base violation does not let the design wander into a problem space beyond certain constraints. The Over-the-Shoulder and Batch modes allow design to occur in a more unconstrained space, and then give indication when constraints are violated or specifications are not met.
Mode Action
Constraint-enforced Don't permit design violations to occur.
Over-the-Shoulder Alert user immediately during design process.
Batch Allow user to explicitly submit design for evaluation.
Do-Nothing Do not perform automated evaluation.
Unconstrained-design evaluation, whether in over-the-shoulder or batch mode, is distinguished from constraint-enforced design. The issues is not as simple as saying that the designer is either empowered by the system or controlled by the system. Both approaches have their trade-offs. Constraint-enforced systems allow for more rapid and correct prototyping while greatly limiting the options and creativity of the designer. These systems often supply a sufficient set of objects along with additional, top-level information such as functional intent with which to work.
Unlike the constrained systems in which information for evaluation is explicitly present, unconstrained systems usually require some abstraction and processing of the lower-level design elements. A bottom-up approach must be taken, some information must be postulated and reconstructed.