Global demographic projections: Future trajectories and associated uncertainty
Demographic projections depict future trends in population size and its distribution by age and sex. Projections are derived using calculations that replicate plausible future changes in a population as a result of fertility, mortality and migration. A central, or middle, scenario depicts a trajectory of change that seems plausible and, overall, more likely than other scenarios. Traditionally, population projections have included two or more alternative scenarios, both to acknowledge the intrinsic uncertainty of the middle scenario and to offer an assessment of the sensitivity of projected trends to changes in underlying assumptions, in particular with regard to fertility. In recent years, statistical models have been used to derive a large number (10,000 or more) of demographic trajectories, each consisting of a series of probabilistic changes over time and, therefore, unique. Typically, for a given outcome or indicator, the median value of the stochastic projection scenarios is chosen to serve as the middle scenario, and the spread around the median provides a probabilistic assessment of the uncertainty of the central forecast. This presentation will examine the most recent set of global population projections from the United Nations (World Population Projects, 2012 Revision) and will review the state of knowledge about their uncertainty.