4 Making sense of data
4.3 Pie charts, bar charts, histograms and line graphs
These are all different ways of representing data and you are likely to be familiar with some, if not all of them. They usually provide a quick summary that gives you a visual image of the data being presented. Below, we have given a brief definition and some ideas of how each can be used, along with a corresponding activity. We suggest that you look out for similar examples in everyday life, and question the information that you see.
4.3.1 Pie charts
A pie chart is a diagram in the form of a circle, with proportions of the circle clearly marked. A pie chart is a good method of representation if we wish to compare a part of a group with the whole group. It gives an immediate idea of the relative sizes of the shares. So, for example, it can be used to consider advertising income. It can also be used to look at, say, shares of market for different brands, or different types of sandwiches sold by a store.
The two short articles opposite use pie charts to compare recruitment advertising volumes in February 1996 and in September 1996 in the UK. Look at them carefully and answer the questions that follow. (The original pie charts are in colour and some of the information is unclear or invisible in black and white. We have therefore added some labels to the pie charts.)
- Which newspaper ran about a third of all advertisements?
- Which two newspapers together ran about a third of all advertisements?
- Where was the data in the pie charts derived from?
- How does it relate to the article?
- What conclusions can you draw from the pie charts and related text?
- The Guardian. Note that one-third is about 33 per cent. In February 1996, it ran 32.6 per cent of the advertisements, and in September 1996, it ran 36.8 per cent of the advertisements. However, see our comments in (5) below.
- The Telegraph and the Sunday Times together. In February 1996, they accounted for 33.2 per cent of the advertisements (17 per cent + 16.2 per cent), and in September 1996, for 32.2 per cent (14.4per cent + 17.8 per cent)
- The data was derived from The Guardian and shows a breakdown of recruitment advertising by column centimetres (an area that is one standard column wide and one centimetre deep). This is the standard format for selling newspaper space. The pie charts show the percentage split and the text in the articles shows what this means in overall volume of advertising.
- Not directly, in either case. The February article simply relates volume of advertising and the fact that, in February 1996; advertising was up by 11 per cent on February 1995. In September 1996, the article relates the increase to the previous 12 months, so it's not possible to compare increases month by month here. It's possible that there was in fact a decrease in advertising space in the year to September 1996. The other interesting feature in the September 1996 article is that it includes a plug for the new internet recruitment advertising programme of The Guardian.
- We think that the source of the graphs and diagrams is relevant. The Guardian, which seems to be leading in terms of advertising space, has provided the details, and has been mentioned in both of the articles. The choice of newspapers shown in the pie charts is interesting, in that most of the broadsheets are mentioned separately for the weekday and Sunday editions (the FT doesn't have a Sunday edition). However, neither the Sunday Telegraph nor The Guardian's Sunday partner, the Observer, are mentioned. This seems a slightly odd way to present the data. There is a possibility that The Guardian has combined the two, so that it shows a higher percentage share being taken by The Guardian than other papers. If you combine the share for The Times and Sunday Times for February 1996, you can see that the total share is 28.7 per cent, and for September, 29.6 per cent; much closer to The Guardian's 32.6 per cent and 36.8 per cent respectively. This may be coincidence, and the Observer may have sales that are not recorded here.
The other small point that interested us is that the two pie charts don't follow the same format. If you look at them closely, you can see there are two differences. First, the newspapers aren't in the same order in both the pie charts and second, in the September chart, a small ‘slice’ of the pie chart is projecting out of it. We think this is probably so that the magazine can fit in the label for the slice, but it has the effect of emphasising the small share of the market of the Independent on Sunday.