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Creating Effective Decision Aids for Complex Tasks

Caroline Clarke Hayes and Farnaz Akhavi

Journal of Usability Studies, Volume 3, Issue 4, August 2008, pp. 152-172

Article Contents


Product Design Processes

In the following sections, we will summarize some of the existing literature describing salient properties and structure of design processes. The models described were developed primarily in the context of mechanical design processes. However, the general characteristics of most complex design processes, whether mechanical, software, or systems, are essentially similar.

Uncertainty is present in all designs (Aughernbaugh & Paredis, 2006), from hand-held computer devices to space station systems. Even when a design is considered to be complete, there may still be uncertainty concerning issues such as the performance of the design under all the conditions to which it may be exposed in its working life, its manufacturing feasibility, or the final cost.

Uncertainty is most prevalent in the early stages of design, also known as conceptual design, when the alternatives under consideration may be little more than quick sketches or brief outlines. At all stages of the design process, designers must repeatedly choose the most promising alternatives for further development. This winnowing of alternatives is known as the down selection process. Conceptual design, down selection, and their relationship to the overall design process are shown in Figure 1. The model of the design process shown in this figure represents the authors'; integration of their own observations with several other design process models (described below) to create a unified design process model.

There is an overall progression in the design process from conceptual design to detail design (Pahl & Beitz, 2006). There is no distinct dividing line between these stages. The design is gradually transformed from a set of sketchy alternatives created during the conceptual design stage into one or more detailed and polished designs that are finalized in the detail design stage. The transformation occurs through many iterations in which they explore, develop, and eliminate many alternatives (Simon, 1985). Multiple iterations of the design process are represented by the helix in Figure 1 (Blanchard & Fabrycky, 2006); each loop of the helix (depicted as a pair of curved arrows) represents one iteration in the design process. The labels on each loop, such as requirements gathering, design review, and down selection, represent some of the activities performed in each iteration. However, design activities rarely proceed in a precise and orderly progression.

Ullman, Dietterrich, and Stauffer (1998) performed protocol studies of mechanical design processes in which designers were asked to create designs to solve real engineering requirements. They found that, in practice, there is much jumping back and forth between steps. From these studies, they developed the Task/Episode Accumulation (TEA) model in which designers incrementally refined and patched design alternatives in a series of design episodes. Each design episode addressed one of six goals: plan, assimilate, specify, repair, document, and verify. These goals can be addressed in almost any order and can be viewed as an alternate way of dividing and describing design activities listed previously.

Figure 1. An iterative model of the design process.

Figure 1. An iterative model of the design process.

The work reported in this article deepens prior work by providing a detailed study of down selection which is the process through which design alternatives are compared and selected for further development.

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