Bar Graphs

Factors That Affect Comprehension

The visual features of a bar graph, such how the bars are organized and how far apart the bars are from one another, can affect comprehension. Bar graph comprehension is often facilitated when the bars are grouped in various ways; for instance, bars that are clustered together along the x-axis or that have the same color are perceived as belonging to the same group and viewers are more likely to make comparisons within rather than across such groups. Additionally, larger effects (i.e., larger height differences between bars) are more salient and easier to perceive.

Because bars are usually spatially segregated in a bar graph, it is typically easier to compare discrete values than continuous values, for which line graphs are better suited. However, Jeff Zacks, Ellen Levy, Barbara Tversky, and Diane Schiano found that discrete height judgments of bars are subject to bias from neighboring elements in bar graphs (such as the presence and height ratio of nearby bars).

[Page 169]Additionally, the knowledge that the user brings to the graph can affect comprehension of bar graphs. One type of knowledge that affects comprehension is graphical literacy, which involves having basic knowledge about how graphs should look and what they are used for. Having graphical literacy would include, for example, knowing that independent or categorical variables are represented along the x-axis and/or legend, whereas the dependent variable is represented along the y-axis. Graph expertise is especially helpful because it can allow the user to compare data more efficiently through the use of mental manipulation and perceptual shortcuts.

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