Types of diagrams

As there is this variety in the types of diagrams we use, we need to think more broadly about what pictures and diagrams are trying to represent. You will encounter three main types of diagrams when studying MST subjects.

1 Pictures or pictorial diagrams that attempt to represent the essential features of a part of reality – for example, diagrams of equipment, molecules or parts of a plant.

2 Diagrams that try to describe interrelationships between ideas, processes or concepts using words, lines and various blobs or boxes.

3 Mathematical diagrams, such as charts and graphs, that are mainly designed to convey mathematical relationships – for example, comparisons over time.

Figure 3.1 Examples of pictorial diagrams. These are different representations of parts of a plant leaf. The details needn’t concern you, but the diagrams differ in scale (the highest magnification, showing a single cell, is at the top), in the amount and type of information conveyed (both structure and processes are represented), and in how realistic they are (the bottom diagram is stylized for clarity)

no labels

fine detail of internal cell structures drawn

all elements drawn to scale

all elements drawn

to scale open stoma cuticle

epidermis

chloroplasts

cuticle photosynthetic

cells

xylem phloem photosynthetic

cells

closed stoma

labels for parts of the leaf

some indication of internal cell

structures

arrows represent pathways for water vapour labels for parts

of the leaf

cuticle xylem (vein ending) mesophyll cells

substomatal cavity

cuticular transpiration

stomatal transpiration elements not

exactly to scale

no indication of internal cell

structures

Pictures, or pictorial diagrams, are a common feature of texts in MST subjects. At their simplest, they are photographs of real objects; at their most complex, they are colourful, fully labelled drawings of the inner workings of organisms or machines. The drawings of bacteria in the Collee article and the diagram of the cress seedling in Chapter 9 (Figure 9.1 on page 244) are good examples. Some other examples, with comments attached, are shown in Figure 3.1.

In nearly all cases, the purpose of the diagram is to illustrate particular features of the world around us. In some cases, the diagrams are used to make the text look pretty or appealing and don’t really add anything to the

understanding of the reader. And even when they’re used more effectively, there is still a need to reflect on the information that is being conveyed.

X Is there a reference to the picture in the accompanying text?

X Is there a title or legend explaining what the picture is about?

X Are there labels on the diagram?

X What are the size and scale of the objects?

X Is the picture a simplified and stylized representation of a complex situation?

Relationship diagrams

Relationship diagrams are largely non-pictorial and aim to represent the structural or organizational features of a situation through combinations of words, lines and arrows, and a wide selection of boxes, blobs and circles.

Examples of this type of diagram include the spray diagram in Chapter 2 (page 49), and the first diagram, entitled ‘Some of the ways … spread’, in the Collee article (page 398). Some other examples are shown in Figure 3.2. In some cases, such as flow diagrams, there may also be numbers, but these diagrams are not primarily used for mathematical relationships (which are often represented using graphs and charts – see the next section).

Relationship diagrams can be broadly divided into those that represent static relationships – for example, maps, classification trees, organization charts, circuit diagrams and influence diagrams – and those that represent a situation over a period of time – for example, flow charts, decision trees, activity sequence diagrams, algorithms and multiple-cause diagrams. You needn’t worry for the moment about what these terms mean – you’ll probably come across examples as you progress through your studies.

Figure 3.2 Examples of relationship diagrams. Don’t be too concerned about the details of the various processes shown here – think more in terms of the variety of different approaches that are possible and the different conventions that are adopted

start

lines or arrows with ‘yes’

or ‘no’ labels from test questions

As with pictorial diagrams, there are questions that must be asked of relationship diagrams.

X Is there a reference or an explanation of the diagram in the accompanying text?

X Is there a title or legend explaining what type of relationship diagram it is, and what situation it represents?

X Is there a key to show what the lines/arrows, boxes/blobs and so on represent?

Graphs and charts

Line graphs, histograms and bar charts are diagrams that show the relationship between two different quantities. For example, in hospital, a patient’s

temperature is often recorded at regular intervals and plotted as a line graph.

This allows medical staff to see at a glance how high the temperature is and how it is changing. You often see graphs and charts in the media summarizing unemployment figures or a company’s profits over the last few months. There are two examples of these types of diagram in the Collee article – ‘The longer food …’ (on page 399) and ‘Numbers of bacteria …’ (on page 401) – as well as several more in this chapter.

Whatever form they take, graphs and charts are used because they summarize numerical information in a way that provides a quick, visual overview but still gives you access to large amounts of data in a condensed format. Whereas a line graph or histogram shows continuous data for all intervals, a bar chart is often used for discrete intervals – say, data from every other year – or when there is more than one value for a particular interval. (There are examples of these types of diagram in Figure 3.3.) A pie chart is simply a way of indicating the proportions of items, with the size of the slices sometimes providing yet more information (see the example on page 179). There is more information on line graphs in Section 10 of Maths Help, particularly in relation to the representation of mathematical relationships rather than just numerical information.

Graphs and charts need to be read very carefully, and the way to do this is described in Section 3.2. Figure 3.3 shows all the features of a typical line graph, histogram and bar chart.

Figure 3.3 Examples of a line graph (top), a histogram (middle) and a bar chart (bottom). Again, you needn’t worry about what data are displayed here (although you may be familiar with the values in the line graph as they’re taken from the Collee article)

vertical label – to show what

is being measured

area of bar represents frequency of item measured Annual yield (tonnes per hectare)

Year

1953 1954 1955 1956 1957 1958 0

Consumptionofproduct ingrams per headper week

Gross weekly income of head of household 0

This brief outline of the types of diagrams you will meet should give you some idea of their purposes and main features. But how and when do you use them? I’ll be looking at both these questions in this chapter, although the emphasis will be on relationship diagrams and graphs and charts. Before I start, I want to emphasize that whenever we take in, think about and express new ideas as part of our learning spiral (see Figure 1.5 on page 22), we describe and represent reality (in words, diagrams or numbers) by making simplifications for some purpose. This has to be so, because reality is so complicated. It is essential to simplify the ‘real world’ in order to be able to describe it or think about it.

In simplifying, we select certain features of a situation – the essentials – to communicate a clear message, without too much clutter obscuring the view.

The view or perspective taken and the selection of features are extremely important in conveying that message. If the photograph of my MP mentioned on page 57 had been taken from 100 metres rather than 10 metres (using the same camera lens), he would probably have been indistinguishable in a crowd of people, or too small to recognize. Similarly, my calendar is great for seeing a week at a time, but my work diary has one day per page – fine for noting down all the meetings I have, but not so good for a long-term view of my workload.

In the two sections that follow, I’ll be looking first at how diagrams can help your studying, and second, at how to use diagrams in your assignments. These topics equate to taking in and thinking through new ideas, and then expressing those newly formed ideas, as described in the learning spiral you met in Chapter 1.