![]() ![]() Second, we modelled CCF50 as a function of educational attainment and contraceptive met need. By contrast, completed cohort fertility rarely increases, making the modelling of CCF50 much more stable. CCF50 is much less affected by the delay of childbearing that occurs as females become more educated, which leads the period measure of the TFR to initially decline to low levels and then increase. First, we modelled completed cohort fertility at age 50 years (CCF50) rather than the TFR. In our study, we improved on UNPD and Wittgenstein forecasts in seven important ways. Estimated in this way, the UNPD predicts TFRs will rebound to approximately 1♷5 in all countries with TFR lower than the replacement level (<2♱). In fitting their global model for low fertility recovery, UNPD has excluded countries with sustained low fertility such as Thailand, South Korea, Canada, and Greece. This blend of statistical models for two of the components of population growth has been used to generate uncertainty intervals (UIs). Beginning in 2010, UNPD adopted a statistical model for the TFR and life expectancy as functions of calendar year and a deterministic model for migration. Structural scenarios were also computed by assuming a fixed difference of 0♵ children in the total fertility rate (TFR) in each time period and country. For many years, UNPD used a deterministic model for fertility, mortality, and migration. Global population projections have been produced by the Population Division of the Department of Economic and Social Affairs of the UN Secretariat (UNPD) since the 1950s. Our alternative scenarios suggest that meeting the Sustainable Development Goals targets for education and contraceptive met need would result in a global population of 6♲9 billion (4♸2–8♷3) in 2100 and a population of 6♸8 billion (5♲7–9♵1) when assuming 99th percentile rates of change in these drivers. China was forecasted to become the largest economy by 2035 but in the reference scenario, the USA was forecasted to once again become the largest economy in 2098. 23 countries in the reference scenario, including Japan, Thailand, and Spain, were forecasted to have population declines greater than 50% from 2017 to 2100 China's population was forecasted to decline by 48♰% (−6♱ to 68♴). By 2050, 151 countries were forecasted to have a TFR lower than the replacement level (TFR <2♱), and 183 were forecasted to have a TFR lower than replacement by 2100. Findings also suggest a shifting age structure in many parts of the world, with 2♳7 billion (1♹1–2♸7) individuals older than 65 years and 1♷0 billion (1♱1–2♸1) individuals younger than 20 years, forecasted globally in 2100. The reference projections for the five largest countries in 2100 were India (1♰9 billion, Nigeria (791 million ), China (732 million ), the USA (336 million ), and Pakistan (248 million ). In the reference scenario, the global population was projected to peak in 2064 at 9♷3 billion (8♸4–10♹) people and decline to 8♷9 billion (6♸3–11♸) in 2100. The global TFR in the reference scenario was forecasted to be 1♶6 (95% UI 1♳3–2♰8) in 2100. Forecast uncertainty intervals (UIs) incorporated uncertainty propagated from past data inputs, model estimation, and forecast data distributions. We estimated the size of gross domestic product for each country and territory in the reference scenario. The model framework was used to develop a reference scenario and alternative scenarios based on the pace of change in educational attainment and contraceptive met need. Net migration was modelled as a function of the Socio-demographic Index, crude population growth rate, and deaths from war and natural disasters and use of an ARIMA model. We modelled age-specific mortality to 2100 using underlying mortality, a risk factor scalar, and an autoregressive integrated moving average (ARIMA) model. Age-specific fertility rates were modelled as a function of CCF50 and covariates. We modelled CCF50 as a time-series random walk function of educational attainment and contraceptive met need. Completed cohort fertility is much more stable over time than the period measure of the total fertility rate (TFR). We developed statistical models for completed cohort fertility at age 50 years (CCF50). We modelled future population in reference and alternative scenarios as a function of fertility, migration, and mortality rates. The Lancet Regional Health – Western Pacific.The Lancet Regional Health – Southeast Asia.The Lancet Gastroenterology & Hepatology.
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