How do projections differ from forecasts? (cont.)

A forecast of future population can be defined as a projection based on assumptions that are predictive and considered to yield the most probable estimates of the future development of a population. Thus, while all forecasts of the population are projections, not all projections are forecasts.

Population forecasts can also be contrasted with population estimates, which are statistics on the current or past size and structure of a population.

A final related term that one encounters in the literature on population projections, particularly in Europe, is "scenario". The term is used to emphasize the "if ... then ..." nature of the calculation by making the assumptions underlying any projection very explicit. Studies using this terminology often present a series of alternative scenarios for the future based on different assumptions. In addition, they may undertake sensitivity analyses of the impact of likely errors in these assumptions. Thus, in informal terms, scenarios can be considered as somewhat more than projections but somewhat less than forecasts.