The reader who has conscientiously worked through the preceding chapter and its examples will have come across many charts and diagrams of varying shapes and forms. In order to refer easily to the form taken by such charts and diagrams, two technical terms will now be introduced.

Frequency distribution. The first term, ‘frequency distribution’, was briefly introduced in chapter 3 and can be illustrated by stating that a table such as table 3.9, where the numbers of schoolboys with heights in different categories are recorded, gives the frequency distribution of the heights of the 100 schoolboys. Thus a frequency distribution merely gives the frequency with which individuals fall into a number of different categories. The interval chosen for the classification is referred to as the group interval, and the frequency in any particular group interval is the group frequency. The manner in which the group frequencies are distributed over the group intervals is referred to as the frequency distribution of the variable.

Table 4.1 is a frequency distribution of the number of incomes that lie between £2,000 and £10,000 in the year ending 5 April 1965. The group intervals in this case do not remain constant over the whole range of the variable (income) but, nevertheless, the resulting distribution is still a frequency distribution. The exercises at the end of chapter 4 give numerous frequency distributions of variables, such as deaths per day, the tensile strength of iron castings, the ages of the population, and the degree of cloudiness at Greenwich.