Pie charts bar charts and linegraphs

It is usual in discussions such as this to distinguish between pie charts, bar charts and line graphs. Pie charts are much rarer in academic articles than are bar charts and line graphs, and probably should be avoided in this context. Pie charts are difficult to label and to read if they contain several segments (see Figure 3.5.2). Further, multicoloured segments do not copy well in black and white.

1950 55 60 65 70 1950 55 60 65 70

Figure 3.5.1 Plotting the same data with different vertical axes can affect the appearance of a graph and the inferences that are drawn from it (fictitious data).

1950 55 60 65 70 1950 55 60 65 70

Figure 3.5.1 Plotting the same data with different vertical axes can affect the appearance of a graph and the inferences that are drawn from it (fictitious data).

Research 14%

Other income 6%

Sales 2%

Hospitality 17%

Government grants 30%

Academic fees 31%

Figure 3.5.2 Pie charts are difficult to label and read when there are segments of different sizes (fictitious data).

Bar charts

Bar charts are easy to construct and are usually clear, but, again, difficulties arise with the labelling if several different components on each measure are presented (in different colours) for separate comparisons. Some authors also seem to forget that what looks clear with separate colours on a computer screen will not look clear in black and white. These difficulties are compounded when authors use computer-based packages to produce three-dimensional (3D) presentations. A sizeable literature now shows that 3D presentations are more confusing than 2D ones (Hartley and Yates, 2001; Mackiewicz, 2007). Figure 3.5.3 shows the differences that ensue, even on a simple chart.

Line graphs

Line graphs are good for showing, say, the performance of two or more groups in different conditions, especially when the data from the different groups vary according to the condition they are in — technically, when there is an 'interaction' between them. Figure 3.5.4 shows such an interaction that it is hard to describe in words and that is also sometimes difficult to detect in a table of numbers. However, line graphs can provide difficulties for readers when there are multiple groups (say, more than three) in multiple conditions (say, more than three).

Tables and graphs thus provide different ways of presenting data, each with their advantages and disadvantages. Writers need to think carefully about which method will be easiest for their readers to understand. Tables are probably best for displaying exact numbers; graphs for displaying trends in the data. As with tables, there are a number of useful texts on graphing techniques (e.g. see Nicol and Pexman, 2003; Tufte, 1983). A key concept introduced by Tufte (1983) is that writers should avoid the use of 'chart-junk' — all those embellishments that add clutter to a display.

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