Probabilistic projections better account for uncertainty than the traditional use of high, medium and low variants. They allow statements about the likelihood of particular demographic regimes.
In a publication in Nature in 1997, Scherbov and others show that the doubling of the world population is unlikely; the odds that the population will double by 2050 are less than a third. However, the doubling of the proportion over the age of 60 is a certainty. Regional differences - understood across 13 world regions - are substantial. Projections show that population growth will probably be most rapid in the Middle East, sub-Saharan Africa and North Africa, with a tripling of the population by 2050 and a quadrupling by 2100 likely (see also Lutz et al., 1998; Scherbov, 1999; Lutz et al., 1999). Their results are based on expert opinion on future trends in fertility, mortality and migration. Experts are familiar with determining the range of plausible levels of demographic indicators that function as parameters in projection models and with determining the probability distributions used in the projection. Monte Carlo micro-simulation techniques are applied to generate random scenarios following predetermined probability distributions of demographic parameters. The final result is a range of demographic futures and a set of scenarios, each with its own probability of occurrence. The method, which is a variant of the Delphi method, was also applied in different contexts - Austria and Germany (Lutz and Scherbov, 1998a, 1998b, 1999a, 1999b).
The research stimulated a global discussion of the uncertainties in population projections. Several countries adopted or are considering adopting the random scenario method or a variant which combines expert opinions and a retrospective analysis of forecasting errors.
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