Introduction

Forecasts are the basis for all forms of planning for the future. Even a complacent assumption that life will continue much as in the past is a form of forecast. Demographic forecasts, in particular, are fundamental to any form of social, economic or business planning. Suppliers of any good or service, from maternity hospitals to the manufacturers of coffins, can only plan ahead if they have some idea how many potential users of their services they can expect to be located where.

Some phenomena can be forecast with perfect accuracy. For example, we know exactly when and where future solar eclipses will occur. Other phenomena, such as the weather, are inherently impossible to forecast with any certainty for more than a few days into the future. It seems unlikely that we will ever to be able to forecast future demographic developments with the certainty that we can eclipses; we certainly cannot yet. Nevertheless, human populations have two fundamental characteristics that reduce uncertainty about how they will develop in the future:

• First, a substantial overlap exists between the current population and the future population. For example, everyone who will be aged 25 or more years in 25 years time has already been born.
• Second, one fundamental aspect of the human condition is that every year that passes we all get exactly one year older until we eventually die.

These two facts constrain possible future developments in a population in ways that often have no equivalent in other fields, such as economic forecasting. Standard methods for making population projections take full advantage of both facts.

This session provides an introduction to population projections and forecasting. It briefly discusses projection methods based on extrapolating population growth before describing the cohort component projection method in more detail.