A Componential Model of Human Interaction with Graphs. V: Using Pie Graphs to Make Comparisons

Research and models of graph reading suggest that the reader's task is an important determinant of the perceptual and cognitive processing components that the reader uses. When people read a pie graph to determine the proportional size of a segment, they apply three processing components: selecting the appropriate mental anchor to which to compare the segment (25%, 50%, or 75%), mentally aligning the anchor to the angular position of the segment around the pie, and mentally adjusting the anchor to match the pie segment size. When a pie graph reader faces a different task, e.g., estimating the ratio of two segments or the difference between two segments, does she use the same processing components to estimate the proportions of A and of B (and then divide one estimate into the other) or does she use a more direct method of mentally aligning the two segments to be compared, then mentally overlaying one on the other (for a ratio) or estimating the spatial difference between the pie segments (for a difference). Two experiments supported the Direct models over the Proportion-based models. The component processes of the Direct models suggest that pie graph designs that eliminated the angular difference between segments being compared should improve performance.