The last chapter has shown how tables can facilitate the reduction of the observer's raw data and material to a form which enables the reader to grasp the essential features portrayed. In this chapter a further stage in this reduction is dealt with in the construction of charts and diagrams, which enable the salient features of a set of data to be picked out and vividly portrayed so that the reader can spot, without detailed study of the individual figures, the features of particular interest. The primary consideration to be borne in mind in the construction of any chart or diagram is clarity, since a confused diagram is of little help and it is probably better to have no diagram at all, than one that is virtually impossible to understand without a great deal of effort on the part of the viewer. To achieve this standard it is essential to decide at the outset on the purpose of the diagram and to exclude all irrelevant matter from consideration.

Broadly speaking, different considerations are involved according to whether the data are concerned with qualitative or quantitative characters. In the former case the study is of some characteristic such as hair colouring, for which it is difficult to have a numerical scale, whereas for quantitative characters, such as the height of schoolboys, it is possible to have a continuous numerical scale whose accuracy is limited only by the inability of the measuring apparatus to record heights to an accuracy of less than about, say ⅛ in.