BUS606: Operations and Supply Chain Management

Meso-level procedural models aim to support the effective generation of good designs by prescribing a systematic design process. A noteworthy early example was published by Evans, who developed a spiral form to highlight the iterative nature of the design process (Fig. 5). Noting that one of the most fundamental characteristics of design is the need to find trade-offs between interdependent factors, Evans argues that design cannot be achieved by following a sequential process alone. He proposes that a structured iterative procedure is adopted to resolve such problems; early estimates are made and repeatedly refined as the design progresses, until such time as the mutually dependent variables are in accord. As the project progresses, these design considerations are gradually refined by repeated attention in the indicated sequence until a balanced solution is reached. At each iteration, the margins available to absorb changes decrease as the interdependencies are gradually resolved, smaller modifications are required, and different methods may be applied to each problem. Evans notes that the effort required and the number of people that can be brought to bear increase as the solution converges.

Other models in this category present the design process as a series of stages, each of which further concretises the design by creating more information about it. This stage-based form is exemplified in the early work of French, (Fig. 6), originally published in 1971. Later models focusing on mechanical design, notably in the work of Hubka and Eder and Pahl et al., prescribe detailed lists of working steps for each stage. These models define how to create the specific forms of information that constitute a mechanical design, progressing from abstract to concrete with the working steps organised such that each stage establishes objectives and constraints for the next. They depict "feedback" between the stages, which indicates the possibility of undesirable rework as well as inter-project and generational learning. Process models of this type are strongly influenced by models of the information structures that define a mechanical system design and its operation. Some years ago, Cross and Roozenburg argued that most had converged upon a consensus form, which is exemplified by Hubka's model (Fig. 7). More recently, some researchers have mapped numerous models onto proposed canonical stages to compare them.

Overall, prescriptive stage-based models promote the idea that following a structured and systematic process will lead to a better result. For example, Pahl et al. state that following their steps ensures that nothing essential is overlooked, leading to more accurate scheduling and resulting in design solutions which may be more easily reused. Although (or because) they are popular, these models have also attracted critique. For example, the models emphasise original design cascading from stakeholder needs, while real-world projects often place strong limitations on the early concept design, with constraints such as existing product platforms and legislative requirements often predetermining the form of the solution. Considering coverage of the models, Gericke and Blessing argue that although procedural models have been adapted to different disciplines, few integrate across them. Other researchers question the pragmatism of mandating a stage-based form. Whitney, for instance, argues that the top–down ideal as represented in these models is clearly desirable, but practical considerations mean that this often merges with a bottom–up fitting-together of existing partial solutions. One reason for this discrepancy is that "a top–down process is very challenging intellectually", because it requires "imagining subassemblies and parts before they are known in any detail". Konda et al. also point out that in collaborative design, participants use different analogies to represent the emerging design and must negotiate solutions, such that the idealised top–down approach proceeding from abstract to concrete may be difficult to maintain. Andreasen et al. summarise some of these concerns when writing that systematic approaches "only give a sparse insight into actual design, whilst giving the impression of rationality, which is not at all present".

Despite perceived limitations, the prescriptive meso-level model forms outlined here have been adapted and applied in many publications proposing discipline-specific design process models. For instance the general form of Evans' spiral model is still in use after more than five decades in fields from naval architecture to software engineering. Stage-based forms may be found in Dym et al., Ullman, Pugh, Roozenburg and Eekels, the VDI guideline 2221, and many other publications.