Bias Due to Visual Illusion in the Graphical Presentation of Information

T. S. Amer , Ph.D.
Northern Arizona University

ABSTRACT:

Graphical displays of information are among the most common decision aids available. Recent research has focused upon the effect that inappropriately prepared graphs may have on decision making. This paper adds to the literature by demonstrating that even a graph that is prepared appropriately may, as a result of visual illusion, bias decision makers who view data from the graph. The investigation of a visual illusion (i.e., an unintended consequence of a correctly depicted graph) is in contrast to prior studies in accounting that have typically examined manipulated graphs (i.e., intentionally distorted, incorrectly depicted graphs). A laboratory experiment was carried out incorporating a two-by-two design to demonstrate the effects of the so-called “ Poggendorff illusion” and the mitigating effect of adding horizontal gridlines. It is shown that decision makers may systematically underestimate or overestimate the values displayed on line graphs. In addition, the results of this research suggest that the bias can be moderated by the inclusion of horizontal gridlines on line graphs. The addition of horizontal gridlines also reduces the variance in some values decision makers view in graphs. The results are shown to have both theoretical and practical implications for information display.

Keyword: Graphs; Poggendorff illusion; bias in decision making; horizontal gridlines.

Data Availability: Data is available from the author upon request.