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Effects of diagram orientation on comprehension

The most popular format of the most common type of diagram in biology textbooks is more difficult to understand than formats that use different orientations.

A study into how well students understand specific diagrams reminds us that, while pictures may be worth 1000 words, even small details can make a significant difference to how informative they are.

The study focused on variously formatted cladograms (also known as phylogenetic trees) that are commonly used in high school and college biology textbooks. Such diagrams are hierarchically branching, and are typically used to show the evolutionary history of taxa.

Nineteen college students (most of whom were women), who were majoring in biology, were shown cladograms in sequential pairs and asked whether the second cladogram (a diagonal one) depicted relationships that were the same or different as those depicted in the first cladogram (a rectangular one). Taxa were represented by single letters, which were either in forward or reverse alphabetical order. Each set (diagonal and rectangular) had four variants: up to the right (UR) with forward letters; UR with reverse letters; down to the right (DR), forward letters; DR, reverse. Six topologies were used, creating 24 cladograms in each set. Eye-tracking showed how the students studied the diagrams.

The order of the letters turned out not to matter, but the way the diagrams were oriented made a significant difference to how well students understood them.

In line with our training in reading (left to right), and regardless of orientation, students scanned the diagrams from left to right. The main line of the cladogram (the “backbone”) also provided a strong visual cue to the direction of scanning (upward or downward). In conjunction with the left-right bias, this meant that UR cladograms were processed from bottom to top, while DR cladograms were processed from top to bottom.

Put like that, the results are less surprising. Diagonal cladograms going up to the right were significantly harder for students to match to the rectangular format (63% correct vs 70% for cladograms going down to the right).

Moreover, this was true even for experts. Of the two biology professors included in the study, one showed the same pattern as the students in terms of accuracy, while the other managed the translations accurately enough, but took significantly longer to interpret the UR diagrams than the DR ones.

Unfortunately, the upward orientation is the more widely used (82% of diagonal cladograms in a survey of 27 high school & college biology textbooks; diagonal cladograms comprised 72% of all diagrams).

The findings suggest that teachers need to teach their students to go against their own natural inclinations, and regardless of orientation, scan the tree in a downward direction. This strategy applies to rectangular cladograms as well as diagonal ones.

It’s worth emphasizing another aspect of these findings: even the best type of diagonal cladogram was only translated at a relatively poor level of accuracy. Previous research has suggested that the diagonal cladogram is significantly harder to understand than the rectangular format. Note that the only difference between them is the orientation.

All this highlights two points:

Even apparently minor aspects of a diagram can make a significant difference to how easily it’s understood.

Teachers shouldn’t assume that students ‘naturally’ know how to read a diagram.


Novick, L., Stull, A. T., & Catley, K. M. (2012). Reading Phylogenetic Trees: The Effects of Tree Orientation and Text Processing on Comprehension. BioScience, 62(8), 757–764. doi:10.1525/bio.2012.62.8.8

Catley, K., & Novick, L. (2008). Seeing the wood for the trees: An analysis of evolutionary diagrams in biology textbooks. BioScience, 58(10), 976–987. Retrieved from

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