A diagram's effectiveness depends on how well it is used by the persons viewing it.
The viewer needs to process it in such way that the end result built up in the viewer's mind closely matches the author's intentions.
Or, the result can be an understanding of the diagram that was not intended by the author.
If the viewer not even knows how to begin processing the diagram, the meaning of it is opaque.

Visual literacy

includes pictorial materials.

To learn from a text, a person needs a level and type of literacy appropriate to the nature of this text.
A history text asks for specific skills, a technical text for other skills.

Visual literacy includes the capacity to read visual displays effectively.
Visual materials pose serious reading challenges to the viewer. Their meaning is not transparent.
Visual materials are not equally accessible to the viewer - compared to text.

Diagrams are not like other pictures: they require diagram knowledge and diagram reading skills.
Not everybody is visually literate with respect to diagrams. Thus, a diagram has to be accompanied by supporting material, in order to help readers process it properly.

The diagram processing task

When we are presented with incomplete information, we try to fill the gaps in that information using knowledge we have stored in long-term memory.

A good diagram is an incomplete set of information: anything considered not of central relevance to the subject matter is removed.
This removal is carried out in a structured, systematic and highly conventionalized way.

So, the viewer needs to be able to reconstruct the situation it depicts:
- identifying relevant information,
- realizing its significance,
- adding information to build up full representation in the viewer's mind.

How diagrams present information

Diagrams use particular techniques to make presentation clearer:
- intentionally distorting or completely altering aspects of reality.
- leaving many things out, omitting much visible material,
- adding graphic markings which can make explanation clearer (e.g. arrows).

For some learner diagrams pose challenges.

Diagrams versus text

With most European languages, reading means going thru a text from left to right and from top to bottom.
Signposts along that way support the reading process: punctuation, headings, font styles, layout, how the chapters follow each other.
The use of these aids is conventionalized and governed by formal rules.

Reading text involves readers in putting together:
- informations presented on the page (external),
- knowledge already inside their long-term memories (internal).

Diagrams have a much lower degree of standardization as texts.
They vary greatly in both:
- types of graphic elements,
- organizational conventions.

Diagrams versus other pictures

Realistic portrayal limits the power to provide explanations. A photograph of a pepper mill tells us little about the functional components and the way these components work together.

Importance of background knowledge

What a persons sees and understands depends upon what the same person already knows about:
- subject matter,
- some of the real-life characteristics of the things represented,
- context of subject matter,
- graphic treatment/relationship used,
- the vocabulary of the diagram,
- conventions related to diagrams,
- significance and relative importance of different types of material constituting the diagram,
- overall organization of the diagram,
- relationships among the diagram's constituents,
- usual exploration pattern of viewers.

Making conventions explicit

Some diagram conventions are quite formal and well-defined, many others are much looser and less standardized,

The designer of diagrams has to consider the influence of these conventions both at the stage of:
- constructing a diagram (the designer's end),
- reading the diagram (the viewer's end).

Subject matter experts underestimate the problems that diagram conventions can cause.
For instructional diagrams, it is usually wise to address conventions explicitly.

Ways to inform viewers about the conventions:
- direct labelling, annotations,
- explanatory keys,
- explanation in the text,
- a preliminary supplementary diagram that explains the conventions.

Especially confusing for beginners in a subject is the use of the same graphic entity for different purposes. Compare: the inconsistent use of arrows.

Why people use diagrams poorly