Effective ways of displaying information

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# 1.3 Pie charts

A pie chart is a way of presenting proportional data in the form of a circle – the ‘pie’. Each ‘slice’ shows its proportion to the whole. The whole itself must be finite and known, for example, the total number of staff in an organisation or the total IT maintenance budget.

Suppose that the staff of an organisation are comprised as shown in Table 4.

## Table 4: The composition of staff in an organisation

Number %
Senior managers 20 10
Other managers 30 15
Clerical 80 40
Total 200 100

You could show this composition in a pie chart like the one in Figure 4.

Figure 4: The composition of staff – a pie chart

The area of a segment (or ‘slice’) of the pie chart corresponds to the proportion that the category occupies in the whole. For instance, the segment marked ‘Other managers’ occupies 15 per cent of the whole pie.

You can use a pie chart when you want to show the components of a whole. It is possible to use a pie chart to illustrate the composition of the staff in an organisation because the data describe the whole organisation. Notice that the percentages add up to 100.

You could also use pie charts to show the composition of staff in an organisation in two (or more) years. Data are shown in Table 5, and data for each year are shown in two pie charts, Figures 5(a) and 5(b).

## Table 5: The composition of staff in an organisation in Year 1 and Year 2

(a) Year 1 (b) Year 2
Number % Number %
Senior managers 20 10 35 14
Other managers 30 15 25 10
Clerical 80 40 130 52
Total 200 100 250 100
Figure 5(a): Composition of staff in Year 1 – a pie chart
Figure 5(b): Composition of staff in Year 2 – a pie chart

The Year 1 pie chart (Figure 5(a)) is the same as Figure 4 because the data are the same. The proportion of senior managers is 10 per cent. Their number increases in Year 2, so in Figure 5(b) which represents that year, they account for 14 per cent of the staff compared with 10 per cent in Year 1. The ‘Senior managers’ segment is proportionately larger. The ‘Other managers’ and ‘Admin’ segments are smaller compared with Year 1, and the ‘Clerical’ segment is larger.

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