Bad Diagrams

All in all, I'm a fan of the New York State Earth Science Regents course and of the accompanying Earth Science Regents exam. New York state enrolls more students in high school level Earth Science than any other state, and confronting the common exam has helped to build a community of practice among New York State Earth Science teachers that is the envy of Earth Science teachers in other states. I especially like the emphasis on building and assessing representational competence--the ability to understand and make inferences from diagrams, maps, profiles, block diagrams, graphs and other visual representations.

However, I have to say that the most recent Earth Science Regents exam (August 20ll) had two really terrible diagrams, so bad that I think they are more likely to sow confusion than illuminate earth processes.

Here is the first one:

When I teach spatial thinking workshops, or interpret diagrams myself, or teach my own students how to interpret diagrams, one of the strongest strategies I know of is to ask what the vantage point or point of view is for the diagram. Is it a map? Is it a profile? Is it a view looking skyward from Earth, and if so in which direction? Is it a view from space, and if so from where? This orienting strategy puts the viewer into position to perceive and interpret the shape of objects and the configurational relationships between objects.

However, this strategy fails here, because this diagram has two separate points of view in the same diagram. The point of view for the Earth and the moon's orbit is from outer space, looking down along a line perpendicular to the plane of the ecliptic, from above the Earth's northern hemisphere. The sun is to the viewer's right. But the point of view for each of the small moons is the view from the Earth, looking towards that moon. The diagram explicitly states that it is "Not draw to scale", but there is no clue in the diagram that the viewing direction as well as the scale varies from place to place within the diagram.

This arrangement breaks faith between the diagram creator and the diagram viewer. Communication via visual representations depends on a set of representational conventions, which are mostly unspoken (at least in most science classes.) One of these is that an individual science diagram can be expected to have one vantage point--except in specific cases where an unusual and conspicuous device, such as an inset, telegraphs a change in vantage point. Sorting out this view direction aberration requires that the viewer already has a sophisticated understanding of spatial relationships of the Sun/Earth/moon system, and thus this diagram is deeply unsuitable for communicating with novices.

Here is the second problematic diagram:

The problem that I see with this diagram ties back to our earlier discussion of "concept-driven visualizations." In that earlier blog post, I made the point that the designer of a concept-driven visualization typically has to choose a single configuration of the Earth system to portray in the diagram--even though the dynamic earth system often has a myriad of configurations and variations. Students tend to remember the specific configuration illustrated, and overgeneralize the normality of that situation.

In this particular diagram, the specific configuration chosen depicts a misconception: the high tide on the right side of the diagram appears to be water which has been pulled towards the moon, which is fine, but the high tide on the left side of the diagram appears to be water which has been pulled towards the sun, which is a problem. Although it is true that there is a tidal bulge underneath the sun, it is of small magnitude relative to the moon's tidal bulge. The tidal bulge on the opposite side of the Earth from the moon is caused by the inertial force of the earth revolving around the center of mass of the Earth/moon system, as explained pretty well in this NOVA interactive. The cause of the tidal bulge opposite the moon is hard to explain and hard for students to grasp, making the alternative conception offered by this diagram--that one bulge is caused by the moon and one bulge by the sun--just that much more attractive.

The best use I can think of for these diagrams is to have students critique them. Identifying and articulating what makes these diagrams sub-optimal could strengthen students' understanding of how diagrams work, an important science process skill sometimes dignified with the term Metarepresentational Competence