This is perhaps the single most important population study for planning purposes. Many important factors of the future situation facing the community on which the planner must advise are derived directly from the population projection. It forms a framework for a great deal of the subsequent work in devising, testing, evaluating and implementing the plan. Most of the important decisions about major land uses and services are derived from population estimates: the demand for water, power and waste disposal facilities; housing, open spaces and schools; the supply of labor; spending power available for the retail trade, the numbers of private cars to be, etc. One must realize that a projection of great sophistication will eventually be used to derive land areas by the application of a standard and that the accuracy or relevance of the standard may be far more important than great refinement in the population projection. Also, one should recall that the cyclical nature of planning, involving regular reviews, gives the opportunity to revise and improve the population projection as part and parcel of that process.
Projections are an extrapolation of historical data (population versus time) into the future. The accuracy of population projections is generally considered directly proportional to the size of the existing population and the historical rate of growth, and inversely proportional to the length of the time projection. In other words:
More confidence in projection
· Short time into the future
· Large population
· Historically high growth
Less confidence in projection
· Long time into the future
· Small population
· Historically low growth
Six methods of population projection arranged in ascending order of accuracy and sophistication
