Probabilistic World Population Projections Based on Expert Opinion

Lutz, W. ORCID: https://orcid.org/0000-0001-7975-8145, Sanderson, W.C., & Scherbov, S. ORCID: https://orcid.org/0000-0002-0881-1073 (1996). Probabilistic World Population Projections Based on Expert Opinion. IIASA Working Paper. IIASA, Laxenburg, Austria: WP-96-017

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Abstract

This paper presents, to our knowledge, the first probabilistic projections of the world population. These projections were carried out as part of the updated 1996 revision (forthcoming in May-June 1996) of "The Future Population of the World. What Can We Assume Today?" (W. Lutz, ed., 1994). Projections are performed at the level of 13 regions to the year 2100. The approach is based on expert judgement about the trend and uncertainty of future fertility, mortality and migration in all the regions. For each of the components a group of experts defined three alternative future paths: low, central, and high. A standard normal distribution is fitted to these assumptions with the central assumption giving the most likely case (mean), and the low and high assumptions giving the range of 90% of all possible cases. Drawing randomly from these distributions, 4000 simulations produced uncertainty distributions for future population size arid age structure. The simulations presented consider both the cases of independence/dependence between regions (whether regions follow the same above or below average trend) and between fertility and mortality trends. One of the many results is that we are able to say now that there is roughly a two-thirds probability that the world population will not double any more in the future. The 95 percent confidence intervals for total world population in 2020 are 7.5-8.3 billion (median: 7.9); in 2050, 8.1-12.0 billion (median: 10.0); and in 2100, 5.7-17.3 billion (median: 10.7).

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