Field of application

A few of the basic characteristics of life tables were mentioned in § 2.11. The purpose of the present chapter is to explain life table technique further, with some illustrations of its use in demography. The essential features of its field of application are:

1. (1) a group of people, all in some defined status or condition; and

2. (2) changes away from this status or condition which gradually deplete the group.

In the example given in §2.11, item (1) represented people alive and item (2) represented deaths.

The technique is to show the numbers remaining in the defined status or condition at any stage, the numbers leaving it at that stage, and some related functions. It can be applied to animals, to plants, or even to inanimate objects – such as railway engines or houses – but, in demographic work, normally only people are concerned. The usual ‘stages’ in the life-table process are successive ages, expressed in years, but examples of other stages that may be encountered are the period of time elapsed since some event, and the number of children borne by a woman.

The nature of the departure from the status or condition may be single (such as death) or multiple (such as marriage and death for bachelors) and is irreversible – for the time being at least. For repeated events, other techniques are preferable.

If sufficient information is available, a life table may be formed very simply by setting forth the complete history of past events as they occurred, but the opportunities for doing so do not often arise.