A major concern of demography is estimating and projecting population, mortality, migration and fertility by age, sex, race, ethnicity and other factors. It is widely used in government and international policy, and research in the health and social sciences. In particular, it is a key input to research and decision-making in global health,
climate change and educational policy.
For the past 10 years, my group has: been working with the United Nations to develop new statistical methods for probabilistic population projections. Traditionally, the UN, like almost all other agencies, produced deterministic population projections, which were supplemented with projections based on different scenarios of demographic changes. This approach has been criticized as lacking validity, and the UN was keen to move beyond it.
For the first time in July 2015, the UN issued official probabilistic population projections for all countries to 2100, using our methods. These projections quantify uncertainty associated with demographic projections, using Bayesian hierarchical models for fertility and mortality. In the probabilistic projection method, uncertainty in future demographic outcomes is assessed by constructing a large sample of future trajectories for these outcomes, for example, total population size. For each year in the future, point projections are given by the median outcome of the sample of trajectories while percentiles of the sample are used to construct prediction intervals.
Some substantive results were summarized by Gerland, Raftery [co-first authors] et al (2014), which was ranked in the top 0.1% of all articles ever published in Science magazine by impact. This article has overturned our understanding of the world’s likely future demographic path. Conventional wisdom had been that world population would peak around the middle of this century and stabilize at around 9 billion, but the new projections indicate that world population stabilization is unlikely this century, and that world population may well exceed 11 billion by 2100.
These new methods and findings have raised many new research questions, both methodological and substantive, which we are now working on.