How do projections differ from forecasts?
A population projection can be defined as:
A computational procedure to calculate population size and structure at one time from population size and structure at another, together with a specification of how change takes place during the interim period.
Several phrases in this definition are worth considering in more detail. Click on each of the italicised phrases in the definition above for an explanation.
Computational procedure
In essence, this means that a population projection is a calculation. It turns one set of numbers into another set of numbers – the answer to the sum. Thus, the only way to get the projection wrong is to get one’s arithmetic wrong. On the other hand, one may carry out the computational procedure correctly without necessarily learning anything useful about the real world from it.
Population size and structure
For some purposes, the only information one needs out of a projection is an estimate of total population size. More often though, the user of the projection also needs to know something about the structure of the projected population, by which we mean its characteristics. In most applications, the projected population is divided up by age and sex. For specific applications, it may also be divided up by additional characteristics such as geographical area, marital status, or economic activity.
At one time from ... another
The crucial point to notice about this part of the definition is that it does not say "population size and structure in the future from population size and structure at present" or even " at a later time ... from an earlier time". In fact, it is possible to project populations backward as well as forward. A number of important applications of projection methods exist in which exactly this is done.
Specification of how change takes place
This specification is the set of assumptions made by the projection. Whether the results of a projection represent changes in any real population will depend on whether these assumptions are applicable to that population. All a projection does is to compute the numerical implications of a set of assumptions.