Graphical Encoding for Information Visualization: An Empirical Study

Research in several areas provides scientific guidance for use of graphical encoding to convey information in an information visualization display. By graphical encoding we mean the use of visual display elements such as icon color, shape, size, or position to convey information about objects represented by the icons. Literature offers inconclusive and often conflicting viewpoints, including the suggestion that the effectiveness of a graphical encoding depends on the type of data represented. Our empirical study suggests that the nature of the users’ perceptual task is more indicative of the effectiveness of a graphical encoding than the type of data represented. 1. Overview of Perceptual Issues In producing a design to visualize search results for a digital library called Envision [12, 13, 19], we found that choosing graphical devices and document attributes to be encoded with each graphical device is a surprisingly difficult task. By graphical devices we mean those visual display elements (e.g., icon color hue, color saturation, flash rate, shape, size, alphanumeric identifiers, position, etc.) used to convey encoded information. Providing access to graphically encoded information requires attention to a range of human cognitive activities, explored by researchers under at least three rubrics: psychophysics of visual search and identification tasks, graphical perception, and graphical language development. Research in these areas provides scientific guidance for design and evaluation of graphical encoding that might otherwise be reduced to opinion and personal taste. Because of space limits, we discuss here only a small portion of the research on graphical encoding that has been conducted. Additional information is in [20]. Ware [29] provides a broader review of perceptual issues pertaining to information visualization. Especially useful for designers are rankings by effectiveness of various graphical devices in communicating different types of data (e.g., nominal, ordinal, or quantitative). Christ  provides such rankings in the context of visual search and identification tasks and provides some empirical evidence to support his findings. Mackinlay  suggests rankings of graphical devices for conveying nominal, ordinal, and quantitative data in the context of graphical language design, but these rankings have not been empirically validated [personal communication]. Cleveland and McGill [8, 9] have empirically validated their ranking of graphical devices for quantitative data. The rankings suggested by Christ, Mackinlay, and Cleveland and McGill are not the same, while other literature offers more conflicting viewpoints, suggesting the need for further research. 1.1 Visual Search and Identification Tasks Psychophysics is a branch of psychology concerned with the "relationship between characteristics of physical stimuli and the psychological experience they produce" . Studies in the psychophysics of visual search and identification tasks have roots in signal detection theory pertaining to air traffic control, process control, and cockpit displays. These studies suggest rankings of graphical devices [6, 7] described later in this paper and point out significant perceptual interactions among graphical devices used in multidimensional displays. Visual search tasks require visual scanning to locate one or more targets [6, 7, 31]. With a scatterplotlike display (sometimes known as a starfield display ), users perform a visual search task when they scan the display to determine the presence of one or more symbols meeting some specific criterion and to locate those symbols if present. For identification tasks, users go beyond visual search to report semantic data about symbols of interest, typically by answering true/false questions or by noting facts about encoded data [6, 7]. Measures of display effectiveness for visual search and identification tasks include time, accuracy, and cognitive workload. A more thorough introduction to signal detection theory may be found in Wickens’ book . Issues involved in studies that influenced the Envision design are complex and findings are sometimes contradictory. Following is a representative overview, but many imProceedings of the IEEE Symposium on Information Visualization 2002 (InfoVis’02) 1522-404X/02 $17.00 © 2002 IEEE portant details are necessarily omitted due to space limitations. 1.1.1 Unidimensional Displays. For unidimensional displays — those involving a single graphical code — Christ’s [6, 7] meta-analysis of 42 prior studies suggests the following ranking of graphical devices by effectiveness: color, size, brightness or alphanumeric, and shape. Other studies confirm that color is the most effective graphical device for reducing display search time [7, 14, 25] but find it followed by shape and then letters or digits [7]. Benefits of color-coding increase for high-density displays [15, 16], but using shapes too similar to one another actually increases search time [22]. For identification tasks measuring accuracy with unidimensional displays, Christ’s work [6, 7] suggests the following ranking of graphical devices by effectiveness: alphanumeric, color, brightness, size, and shape. In a later study, Christ found that digits gave the most accurate results but that color, letters, and familiar geometric shapes all produced equal results with experienced subjects . However, Jubis [14] found that shape codes yielded faster mean reaction times than color codes, while Kopala  found no significant difference among codes for identification tasks. 1.1.2 Multidimensional Displays. For multidimensional displays — those using multiple graphical devices combined in one visual object to encode several pieces of information — codes may be either redundant or non-redundant. A redundant code using color and shape to encode the same information yields average search speeds even faster than non-redundant color or shape encoding [7]. Used redundantly with other codes, color yields faster results than shape, and either color or shape is superior as a redundant code to both letters and digits [7]. Jubis [14] confirms that a redundant code involving both color and shape is superior to shape coding but is approximately equal to non-redundant color-coding. For difficult tasks, using redundant color-coding may significantly reduce reaction time and increase accuracy [15]. Benefits of redundant color-coding increase as displays become more cluttered or complex . 1.1.3 Interactions Among Graphical Devices . Significant interactions among graphical devices complicate design for multidimensional displays. Color-coding interferes with all achromatic codes, reducing accuracy by as much as 43% [6]. Indeed, Luder [16] suggests that color has such cognitive dominance that it should only be used to encode the most important data and in situations where dependence on color-coding does not increase risk. While we found no supporting empirical evidence, we believe size and shape interact, causing the shape of very small objects to be perceived less accurately. 1.1.4 Ranges of Graphical Devices. The number of instances of each graphical device (e.g., how many colors or shapes are used in the code) is significant because it limits the range or number of values encoded using that device . The conservative recommendation is to use only five or six distinct colors or shapes [3, 7, 27, 31]. However, some research suggests that 10  to 18 [24] colors may be used for search tasks.  Non-integration Tasks. Later research has focused on how humans extract information from a multidimensional display to perform both integration and non-integration tasks [4, 26, 27]. An integration task uses information encoded non-redundantly with two or more graphical devices to reach a single decision or action, while a non-integration task bases decisions or actions on information encoded in only one graphical device. Studies [4, 30] provide evidence that object displays, in which multiple visual attributes of a single object present information about multiple characteristics, facilitate integration tasks, especially where multiple graphical encodings all convey information relevant to the task at hand. However, object displays hinder non-integration tasks, as additional effort is required to filter out unwanted information communicated by the objects. 1.2 Graphical Perception Graphical perception is “the visual decoding of the quantitative and qualitative information encoded on graphs,” where visual decoding means “instantaneous perception of the visual field that comes without apparent mental effort” [9, p. 828]. Cleveland and McGill studied the perception of quantitative data such as “numerical values of a variable...that are not highly discrete...” [9, p. 828]. They have identified and empirically validated a ranking of graphical devices for displaying quantitative data, ordered as follows from most to least accurately perceived [9, p. 830]: Position along a common scale; Position on identical but non-aligned scales; Length; Angle or Slope; Area; Volume, Density, and/or Color saturation; Color hue.  and thus call for attention to appropriate use of each graphical device. In his discussion of graphical languages, Mackinlay [17] suggests three different rankings of the effectiveness of various graphical devices in communicating quantitative (numerical), ordinal (ranked), and nominal (non-ordinal textual) data about objects. Although based on psychophysical and graphical perception research, Mackinlay's rankings have not been experimentally validated [personal communication].  These studies make it clear that no single graphical device works equally well for all users, nor does an