Elementary Statistics (1+1)

5.1.11 Choice of a Suitable diagram

Which diagram out of several ones to select in a given situation is a ticklish problem. The choice would primarily depend upon two factors, namely : (i) the nature of the data; (ii) the type of people for whom the diagram is meant. On the nature of the data would depend whether to use one-dimensional, two-dimensional or three-dimensional diagram, and if it is one-dimensional, whether to adopt the simple bar or subdivided bar, multiple bar or some other type. As already stated, a cubic diagram would be preferred to a bar if the magnitudes of the figures are very wide apart. The type of people for whom the diagram is intended must also be considered. For example, for drawing attention of an uneducated mass, pictographs and cartograms are more effective than cubes, circles, etc. Different types of diagrams such as bars, rectangles, cubes, pictographs, pie charts, have specialised uses. However, bar diagrams are the most popular in practice. There are different types of bars and the appropriate type of bar chart can be choosen on the following basis :

a) Simple bar charts should be used where changes in totals are required to be conveyed.

b) Component bar charts are more useful where changes in totals as well as in the size of components figures (absolute ones) are required to be displayed.

c) Percentage composition bar charts are better suited where changes in the relative size of component figures are to be exhibited.

d) Multiple bar charts should be used where changes in the absolute values of the components figures are to be emphasised and the overall total is of no importance.

However, multiple and component bar charts should be used only when there are not more than three or four components, as a large number of components make the bar charts too complex to enable worthwhile visual impression to be gained. When a large number of components have to be shown, a pie chart is more suitable.

A pie chart is particularly useful where it is desired to show the relative proportions of the figures that go to make up a single overall total. Unlike bar charts it is not restricted to three or four component figures although its effectiveness tends to dwindle with more than seven or eight components.

However, pie charts cannot be used effectively where a series of figures is involved, as a number of different pie charts are not easy to compare. Nor should changes in the overall total be shown by changing the size of the ‘pie’.

Occasionally, circles are used to represent size. But it is difficult to compare them and they should not be used when it is possible to use charts. This is because it is easier to compare the lengths of lines or bars then to compare areas or volumes.

Cubes should be used in those cases where the difference between the smallest and largest values to be represented is very large. In other cases cubes should not be used because comparison is too difficult with the help of cubes.

Pictographs and cartograms are very elementary form of visual presentation. However, they are more informative and more effective than other forms for presenting data to the general public who, by and large, neither possess much ability to understand nor take interest in the less attractive forms of presentation. The pictograph is admirably suited to the illustrations of exhibits or articles in newspapers and magazines or for dressing up annual reports. Cartograms or statistical maps are particularly effective in bringing out the geographical pattern that may be concealed to the data.