Bias in Proportion Judgments with Pie Charts: The Cyclical Power Model

Abstract

Subjects were shown pie charts with tickmarks at two, three, four, or eight evenly-spaced locations. The tickmarks were located around the edge of the pie, on a line where the subject responded, or both. A cyclical power model based on Stevens' law was used to account for the observed bias patterns. The model proposes that subjects use reference points to constrain the range of values used in the judgment. When more than two reference points are used, the model predicts multiple cycles of bias. The results showed that increasing the number of reference points on the response line from two to four changed the number of bias cycles, while the estimated Stevens exponent remained approximately constant at 0.8. The model can be used as a tool to assess how graphs, dials, and meters are read.