Truth or Square: Aspect Ratio Biases Recall of Position Encodings

TL;DR

This paper investigates how aspect ratio biases in bar charts influence people's memory of data positions, revealing that aspect ratios cause over- or underestimation due to a bias toward prototypical square shapes.

Contribution

It explains conflicting previous findings by linking aspect ratio to bias in recalling bar positions and demonstrates this effect across multiple empirical studies.

Findings

Aspect ratio affects recall bias in bar chart reproductions.

Wide bars tend to be overestimated, tall bars underestimated.

Memory-based reproductions show similar bias patterns.

Abstract

Bar charts are among the most frequently used visualizations, in part because their position encoding leads them to convey data values precisely. Yet reproductions of single bars or groups of bars within a graph can be biased. Curiously, some previous work found that this bias resulted in an overestimation of reproduced data values, while other work found an underestimation. Across three empirical studies, we offer an explanation for these conflicting findings: this discrepancy is a consequence of the differing aspect ratios of the tested bar marks. Viewers are biased to remember a bar mark as being more similar to a prototypical square, leading to an overestimation of bars with a wide aspect ratio, and an underestimation of bars with a tall aspect ratio. Experiments 1 and 2 showed that the aspect ratio of the bar marks indeed influenced the direction of this bias. Experiment 3 confirmed that this pattern of misestimation bias was present for reproductions from memory, suggesting that this bias may arise when comparing values across sequential displays or views. We describe additional visualization designs that might be prone to this bias beyond bar charts (e.g., Mekko charts and treemaps), and speculate that other visual channels might hold similar biases toward prototypical values.