Affording Design, Affording Redesign - Principia Designae: Pre-Design, Design, and Post-Design - page 89

shortest paths. We called this a sequential bias, to treat all connections as linear and
ordered. Systems diagrams bear superficial resemblance to more familiar route
maps, which are appropriately interpreted sequentially. The inference errors of
omission indicated a second bias, a reading order bias. Students tended to list
shortest paths in reading order, from upper left to lower right, and to miss the later
paths. Importantly, students who did master the bus in their sketches made more
correct inferences from them. In both cases, students are bringing previous habits
for interpreting marks on pages to design; the previous habits impede interpreta-
tions of systems design sketches. The data supporting these conclusions depended
on characterizing the diagrams as well as the inferences and comparing the two.
Other work on diagrams corroborates these finding. Research on describing
environments represented as sketch maps revealed a reading order bias (Taylor
and Tversky 1992 ) . When the environments had a natural beginning, like an
entrance, people tended to start their descriptions at the entrance. But when envi-
ronments did not have a natural beginning, people began their descriptions at the
upper left. Another striking example is a resistance to diagram cycles as circles,
instead preferring to represent them linearly. When students are asked to make a
representation on paper to show the seasons or to show the water cycle, most students
make linear arrays rather than circular ones (Kessell and Tversky 2007 ) . That is, they
see the processes as having a beginning, middle, and end rather than endless
repetition. Put together, the findings suggest that students bring habits from everyday
spatial reasoning and interactions with text and with simpler diagrams, such as maps,
to the creation and understanding of sketches of more complex and subtle ideas. The
implication is that creation and interpretation of sketches should be carefully taught.

7.2.3 Ideas

It has become clear that the core structure of a broad range of sketches is nodes and
links, where nodes are typically entities, concrete or abstract, places or ideas, and
links are the connections among them. Another, more abstract, way of putting it is
that the nodes are nouns and the links are predicates; nouns and predicates of course
are the basic elements of language. In fact, the node-link structure is also commonly
used for abstract ideas, where the nodes are virtual entities, concepts, and the links
are relations among them. The applications are too many to list, from concept maps
to encourage order in ideas in school children to flow charts to decision trees to
visualizations of the web and Facebook networks. The ubiquity of node-link dia-
grams suggests that they reflect core qualities of human thought. Two nodes and a
link is a minimal diagram; a subject and a predicate a minimal sentence.