Introduction
This session follows on from the session on Population projections: concepts and methods 
in the 'Introduction to demographic analysis' module. The earlier session discusses the difference between projections and forecasts. It then provides an introduction to the calculation of projections based on (a) the extrapolation of trends in total population size and (b) the cohort-component method. This session assumes that the reader has a sound understanding of that material.
The current session is concerned less with the computational aspects of conducting a population projection and more with the practice of population forecasting and its limitations. It has two themes: diversity and uncertainty.
The key point that it makes about diversity is that forecasts are carried out for many different purposes and that different users of forecasts require different information about the future population. Because no one set of projection outputs can be defined that will serve the need of all users, it is impossible to define a single recommended way of forecasting the future population. Instead, the methods used to project a population must be adapted to the application.
The key point that the session makes about uncertainty is that population forecasts, like forecasts in many other fields, can never be completely accurate. The future of any population is to an extent unknowable. Because forecasting error is inevitable, it is important to understand its implications. Thus, the session discusses how inaccurate population forecasts are likely to be, how this differs across different outputs from a population projection, and what determines the size of these errors.