In this report we describe the concept of the proximity compatibility principle (PCP) and demonstrate its relevance to display design: Displays relevant to a common task or mental operation (close task or mental proximity) should be rendered close together in perceptual space (close display proximity). Different forms of task proximity are discussed, as are the different information-processing mechanisms that underlie the effects of the several different design manipulations of display proximity. Experimental data that support this process-based elaboration of PCP are then reviewed in design contexts relating to aviation, graphs, display layout, and decision aiding.
Consider the task confronting the designer of an interface for a complex system. Multiple sources of information are provided by the system, and task analysis, coupled perhaps with visual scanning analysis of operators using a prototype, has provided some insight regarding the nature of the operator's information needs: which sets of indicators need to be compared or combined (used simultaneously), which are to be used in sequence to perform a certain task, which may rarely be used in sequence but still pertain to common system elements, and which have nothing to do with one another.
This article addresses the issue of where these different sources should be placed with respect to one another and how they should be organized. With an earlier generation of electromechanical indicators, the issue was simply one of spatial location on a two-dimensional (2D) display panel. However, the greater flexibility of electronic display options enabling display integration, color, and multifunctionality increases the flexibility of design and leads to a far more complex meaning assigned to the concept of "where."
We describe the proximity compatibility principle as one guideline to use in determining where a display should be located, given its relatedness to other displays. The PCP depends critically on two dimensions of proximity or similarity: perceptual proximity and processing proximity. Perceptual proximity (display proximity) defines how close together two display channels conveying task-related information lie in the user's multidimensional perceptual space (i.e., how similar they are). Thus two sources will be perceptually more similar (in closer proximity) if they are close together, share the same color, use the same physical dimensions (e.g., both use orientation or length), or use the same code (e.g., both are digital or both are analog). For the designer, perceptual proximity is influenced by variation in where and how information sources are displayed, so this may be also referred to as display proximity.
Mental or processing proximity defines the extent to which the two or more sources are used as part of the same task. If these sources must be integrated, they have close processing proximity. If they should be processed independently, their processing proximity is low. The principle proposes a compatibility between these two dimensions. If there is close processing proximity, then close perceptual proximity is advised; conversely, if independent processing is required, distant perceptual proximity is prescribed.
Certain aspects of the PCP are familiar to the design community. For example, the classic principle of functional grouping dictates close proximity between functionally related instruments (Bailey, 1989; Bonney and Williams, 1977) and has been successfully practiced in the layout of aircraft instruments. However, we go beyond this principle in three respects. First, as noted, we broaden the concept of 2D space to include concepts of perceptual space and perceptual "closeness" or similarity (Garner, 1970, 1974). Second, we consider not only the benefits of closeness but also its costs. Third, we attempt to relate the principle to a number of different psychological or information-processing mechanisms that are responsible for the PCP effect. …