Precision refers to the relevance of details with respect to the purpose of the prototype 1 . For example, when sketching a dialog box, the designer specifies its size, the positions of each field and the titles of each label. However not all these details are relevant to the goal of the prototype: it may be necessary to show where the labels are, but too early to choose the text. The designer can convey this by writing nonsense words or drawing squiggles, which shows the need for labels

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... specifying their actual content. Although it may seem contradictory, a detailed representation need not be precise. This is an important characteristic of prototypes: those parts of the prototype that are not precise are those open for future discussion or for exploration of the design space. Yet they need to be incarnated in some form so the prototype can be evaluated and iterated. Iterative design involves multiple design-implement-test loops 2 , enabling the designer to generate different ideas and successively improve upon them. Prototypes support this goal by allowing designers to evaluate concrete representations of design ideas and select the best. Prototypes reveal the strengths as well as the weaknesses of a design. Unlike pure ideas, abstract models or other representations, they can be contextualized to help understand how the real system would be used in a real setting. Because prototypes are concrete and detailed, designers can explore different real-world scenarios and users can evaluate them with respect to their current needs. Prototypes can be compared directly with other, existing systems, and designers can learn about the context of use and the work practices of the end users. Prototypes can help designers (re)analyze the user's needs during the design process, not abstractly as with traditional requirements analysis, but in the context of the system being built.