In the opening stages of a statistical inquiry the investigator will need to collect a large amount of raw material or data from which to extract the quantities relevant to the purposes that he has in mind. Thus the market research investigator may have to collect data on consumer preferences after four different sets of advertisements have appeared; the botanist may have to spend several days in a grassland area counting the number of shoots of Solidago glaberima per square foot; the traffic investigator may have to count traffic at a busy crossing; the agriculturalist may have to collect data concerning the quantity of fertiliser applied to wheat crops on all the farms in Sussex. The method and care given to the collection of this raw material is important. The strength of a chain lies in the strength of its weakest link and it is useless to reach intricate conclusions from insufficient or inaccurate data. Before making any form of elaborate analysis it is essential to know the limitations and accuracy of statistical material and to be aware of the kind of errors that can arise. In this chapter two of the most common sources of data will be considered in some detail. These are:

1. (i) Questionnaires. The data here are obtained by forms designed by the statistician and completed by the general public.

2. (ii) Observations. The data here are collected by the investigator himself recording the results of a series of observations but not necessarily relying on the public at large for his information.

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.