Illustrations 109
interpreted, and draw attention to features you wish the reader to note, even if
you have discussed these in some detail in the text.- Don’t cram in too much detail. When I ask students their view on the functions of
tables, for example, they often reply that it is to record data such as experimental
readings in a systematic way. This being so, a table might have to contain large
amounts of data, perhaps extending over several pages. In my view such data
should not go in the main text, but rather in an appendix. A table in the main text
must be a complementary channel of communication, and illustrative rather than
exhaustive—that’s why they’re called illustrations. - Reserve the use of illustrations for things that are important. The reader will fo-
cus on them and assume that they are the most significant part of your work; use
of illustrations for minor outcomes can skew the reader’s understanding of your
argument. - Put in a table only when the patterning obtained by arranging things in rows
and columns tells the reader something better than or different from a written
description. If the data in your table seems to you to demonstrate some trend or
correlation, you should consider displaying the trend by means of a graph, and
banishing the figures to a table in an appendix. - A diagram should be a net aid to understanding. If the work of explaining a dia-
gram that illustrates, say, risk factors in diet is more work than simply explaining
the risk factors, then the diagram is an unhelpful burden and should be discarded.
(It can be helpful, though, to develop such diagrams for your own benefit, as
they can clarify your understanding and help you focus in on what you are try-
ing to say.) That said, never overlook the possibility that a pertinent diagram can
greatly improve your presentation.
There are three kinds of figures: diagrams, graphs, and images such as photographs.
This book is not the place to give detailed advice on preparation of such materials,
but it is important that you be aware of some general principles.
Some authors like to describe aspects of their work as line diagrams made up of
boxes, circles, arrows, labels, and so on. Such diagrams can be a powerful way of
explaining relationships, but they are inevitably simplifications of complex situa-
tions, and may degenerate into caricatures. A diagram may be a useful way of il-
lustrating the biology of food digestion, for example, but that doesn’t mean that a
diagram is the right way to show the connection between the sociopolitical factors
that control foodstuff quality.
Graphs are used to demonstrate trends or correlations; so you need to think care-
fully about what you are trying to demonstrate. Usually you will be either confirm-
ing an established model or developing a new one, and you should have this in mind
when plotting your graph. A common error is the introduction of an extraneous
variable. In a striking example of this, the authors were trying to demonstrate that
reducing the lead emissions into urban air from the combustion of petrol would
reduce the lead concentration in the blood of children. The data available to them
were figures taken in the US over the five-year period 1976–1980. The writers plot-
ted both the quantity of lead in petrol sold in major cities and the lead content of