Box, Line, and Midgap Plots: Effects of Display Characteristics on the Accuracy and Bias of Estimates of Whisker Length

Abstract

Examined here were the accuracy and bias of estimates of whisker length based on box, line, and midgap plots. For each type of graph, a different sample of undergraduates viewed 48 single-plot graphs. For each plot, subjects were given the length of an interquartile spread and asked to estimate the length of a whisker. Plots varied in spatial orientation (horizontal or vertical), interquartile spread, the ratio of whisker length to interquartile spread, and whisker judged. Overall, estimates of whisker length for box and line plots were more accurate and less biased than those for midgap plots. Interquartile spread, the ratio of whisker length to interquartile spread, and the interaction of these two factors significantly influenced both accuracy and bias. Boxplots displayed a predicted pattern of over- and underestimation. We discuss how the present results complement related work of others (e.g., Cleveland & McGill, 1984 ) on depiction of quantity. We conclude that midgap plots are less optimal displays than box and line plots.