In addition to the medium-variant projection based  on probabilistic methods which provide 80 and 95 per cent prediction intervals, the 2019 Revision includes nine different projection variants (see table below) that convey the sensitivity of the medium-variant projection to changes in the underlying assumptions, and to explore the implications of alternative future scenarios of population change. The results for these additional projection variants are available from the Download Center for the standard projection results in Excel file format as extra worksheets in each workbook file), or ASCII CSV file format. Probabilistic projection results are available as a separate set of files, see Download > Probabilistic Projections and Figures.

Medium-variant projection: in projecting future levels of fertility and mortality, probabilistic methods were used to reflect the uncertainty of the projections based on the historical variability of changes in each variable. The method takes into account the past experience of each country, while also reflecting uncertainty about future changes based on the past experience of other countries under similar conditions.  The medium-variant projection corresponds to the median of several thousand distinct trajectories of each demographic component derived using the probabilistic model of the variability in changes over time. Prediction intervals reflect the spread in the distribution of outcomes across the projected trajectories and thus provide an assessment of the uncertainty inherent in the medium-variant projection.

Fertility variants: Five of those variants differ only with respect to the level of fertility, that is, they share the same assumptions made with respect to mortality and international migration. The five fertility variants are: low, medium, high, constant-fertility and instant-replacement-fertility. A comparison of the results from these five variants allows an assessment of the effects that different fertility assumptions have on other demographic parameters. The high, low, constant-fertility and instant-replacement variants differ from the medium variant only in the projected level of total fertility. In the high variant, total fertility is projected to reach a fertility level that is 0.5 births above the total fertility in the medium variant. In the low variant, total fertility is projected to remain 0.5 births below the total fertility in the medium variant. In the constant-fertility variant, total fertility remains constant at the level estimated for 2015-2020. In the instant-replacement variant, fertility for each country is set to the level necessary to ensure a net reproduction rate of 1.0 starting in 2020-2025. Fertility varies slightly over the projection period in such a way that the net reproduction rate always remains equal to one, thus ensuring the replacement of the population over the long run.

Mortality variants: A constant-mortality variant, and a “no change” variant (i.e., both fertility and mortality are kept constant) are available. The constant-mortality variant uses the same fertility assumption (medium fertility), and international migration assumption as the medium variant. Consequently, the results of the constant-mortality variant can be compared with those of the medium variant to assess the effect that changing mortality has on various population quantities.

Migration variants: similarly, the zero-migration variant differs from the medium variant only with respect to the underlying assumption regarding international migration. Therefore, the zero-migration variant allows an assessment of the effect that non-zero net migration has on various population quantities. The “no change” variant has the same assumption about international migration as the medium variant but differs from the latter by having constant fertility and mortality. When compared to the medium variant, therefore, its results shed light on the effects that changing fertility and mortality have on the results obtained.

The momentum variant illustrates the impact of age structure on long-term population change(United Nations, 2017). The variant combines elements of three existing variants: the instant-replacement-fertility variant, the constant-mortality variant, and the zero-migration variant.

 

For further details, see also the report World Population Prospects 2019: Methodology of the United Nations Population Estimates and Projections.