On Varying the Detectability of Symbols to Encode Data Uncertainty
Visual variables are properties of an image, such as the size or shape of a mark, that can be manipulated systematically to convey information. However, there is another, more fundamental property of images which has not been yet been examined as a potential carrier of uncertainty information in maps: detectability. The detectability of a symbol is simply how easily it can be noticed by a viewer, and it depends on its size and the brightness contrast it has with its background. Both size and brightness (often referred to as color value) are visual variables, and while experimentally manipulating these to represent uncertainty, previous researchers have at the same time been altering detectability, though they have not explicitly addressed it. Thus, it is not clear if the experimental results of earlier authors can be attributed simply to the fact that they changed the size or color value of symbols, or if in fact their results are better explained by how those symbols differed from their visual context. Extracting useful guidance from prior experimental work requires an answer to this question. This thesis examines the role of detectability ? of visual context ? in creating a perception of uncertainty in map readers. In order to examine potential relationships between the physical phenomenon of detectability (which relies on changes in light striking the retina) and the cognitive notion of certainty, this thesis puts forth two hypotheses: 1) In absence of a legend to guide them, map readers will tend to interpret more detectable symbols as representing more certain data, rather than less certain data. 2) Readers will more accurately extract uncertainty information from maps that encode more certain data by more detectable symbols than they will from maps that encode certainty with the opposite scheme (less certain is more detectable).