You can get a lot of information from a simple bar graph, but to what extent does the arrangement of the bars matter? You can find great commentary about good design, but what about a nice clean experiment? Martin H. Fischer led a team that asked participants to indicate if a given relationship was true or false, based on a variety of different bar graphs. For example, is A > B in this graph?
And what about this one?
If you are like most people, it was easier to confirm that A was larger than B in the first graph — where the bars were oriented vertically. In addition to manipulating whether the bars were vertical or horizontal, Fischer et al also looked at where the numbers fell along the number line. In the following graphs, the absolute difference between the black and white bars is a constant, but it might take you a minute to really see that.
In previous research, Fischer had found that participants are faster to make left responses to smaller numbers, and right responses to larger numbers. In other words, when confronted with two numbers, you are faster to identify the leftmost one as the smaller, and the rightmost one as the larger. This suggests that your underlying mental number line starts with small numbers on the left — which is certainly the way most rulers are printed. Given this left-right coding, it would follow that horizontally oriented bars would be easier to read. However, that’s not what Fischer et al actually observed: participants were significantly faster with vertically oriented bars depicting positive or negative values. The orientation difference didn’t hold for graphs depicting mixed pairs (one positive, one negative).
Participants were also significantly faster at making decisions regarding positive numbers, regardless of orientation. What is particularly interesting about these results is that our underlying mental number line, which seems to run from left to right, is not leading to an advantage for horizontally oriented bars. Instead, there is a suggestion that increased vertical height corresponds to “more” — something which might be more universal than a number line that matches Western reading habits.
One final, personal note about gridlines (those lines that extend across the graph for each scale mark): Yuck! Fischer et al included them in the stimuli for their experiment, and so I included them here, but I bet they get in the way as much, if not more, than bar orientation. Clearly, there is more work to be done.
Fischer, M. H., Dewulf, N. & Hill, R. L. (2005). Designing bar graphs: Orientation matters. Applied Cognitive Psychology, 19, 953-962.