Joint Probabilistic Projection of Female and Male Life Expectancy

AbstractBACKGROUNDThe United Nations (UN) produces population projections for all countries every two years. These are used by international organizations, governments, the private sector and researchers for policy planning, for monitoring development goals, as inputs to economic and environmental models, and for social and health research. The UN is considering producing fully probabilistic population projections, for which joint probabilistic projections of future female and male life expectancy at birth are needed.OBJECTIVEWe propose a methodology for obtaining joint probabilistic projections of female and male life expectancy at birth.METHODSWe first project female life expectancy using a one-sex method for probabilistic projection of life expectancy. We then project the gap between female and male life expectancy. We propose an autoregressive model for the gap in a future time period for a particular country, which is a function of female life expectancy and a t-distributed random perturbation. This method takes into account mortality data limitations, is comparable across countries, and accounts for shocks. We estimate all parameters based on life expectancy estimates for 1950-2010. The methods are implemented in the bayesLife and bayesPop R packages.RESULTSWe evaluated our model using out-of-sample projections for the period 1995-2010 and found that our method performed better than several possible alternatives.CONCLUSIONSWe find that the average gap between female and male life expectancy has been increasing for female life expectancy below 75, and decreasing for female life expectancy above 75. Our projections of the gap are lower than the UN's 2008 projections for most countries and thus lead to higher projections of male life expectancy.(ProQuest: ... denotes formulae omitted.)1. IntroductionEvery two years, the Population Division of the United Nations (UN) publishes the World Population Prospects (WPP), reporting population estimates and projections for all coun- tries. Experts then use these projections to monitor population trends and as inputs to economic and environmental models. The UN's population projections form the basis of many nations' and regions' development plans (Heilig et al. 2010).Since 1980, each edition of the WPP has included three projection variants: low, medium, and high. The medium variant is intended to represent the most likely population size given past trends and expert knowledge. The low and high variants are then generated using different fertility assumptions: total fertility is taken to be half a child either below or above that used to generate the medium scenario. Thus the variation in fertility allowed by this method is insensitive to either the level of fertility in a particular country or its trend. The assumptions about mortality and international migration are the same in all three variants.Deterministic population projections are unable to take into account the full variabil- ity in demographic processes across countries or to indicate a range of future population outcomes. As a result, the UN is considering issuing probabilistic population projections as an alternative to their standard variants of deterministic projections. As WPP projec- tions are based on the cohort-component method, probabilistic projections of the main de- mographic processes affecting national populations (fertility, mortality and international migration) are required to produce probabilistic population projections for all countries.Probabilistic projections of fertility for all countries have already been developed by Alkema et al. (2011) and implemented in WPP 2010 (United Nations 2011). The medium variant in WPP 2010 corresponds to the median of the many possible future fertility paths for each country generated by the Alkema et al. (2011) model.Here we are concerned with the projection of mortality. …