Comment on “A Model for Studying Display Methods of Statistical Graphics”

Abstract

In his opening section Cleveland writes provocatively: is not true that the selection of statistical information is the only critical part and that virtually any method of display suffices. This is so aptly demonstrated that even a reader who only browses the article benefits from looking at the sample graphs. The good/bad examples show how important properties of time curves are disclosed by selecting the right aspect ratio, and how grid lines can greatly facilitate comparisons between displays. Another example shows how data from experimental design with as many as three factors can be shown in a simple two-dimensional display so that one sees the main effects and interactions at work before turning to abstract p values and F ratios based on often doubtful assumptions. Why do some graphs work and others do not? There are scientific reasons for it that have to do with the properties of the human visual system, and these reasons are the focus of the article. However, the goal is not to contribute to the science of visual information processing, but rather, as the author states, it is an engineering one: It is to derive rationales for the design of better display methods. 1. PARALLEL AND SERIAL VISUAL PROCESSES As many authors in vision research, Cleveland feels that there are two profoundly different modes of visual processing. He calls them pattern and table lookup. Pattern perception consists of detection of graphical elements, grouping them so to detect structure and compare quantitative physical aspects of different elements. Table look-up means to actually decode quantitative or categorical information in the data, mainly by relating them to the scales given in the display. This distinction is reminiscent of, but not identical to, a common distinction in vision research: There is much empirical evidence to support a primary, fast, parallel visual process that instantaneously integrates information over the whole visual field that has been called preattentive vision (Julesz 1981; 1984). There is also evidence that supports a secondary visual process that can only process a small part of the visual field at a time, and that therefore scans the field in a slow and serial way. This process has been called