What are population projections of official statistics?
Population projections are based on certain assumptions regarding the development of major components of population change, namely fertility, mortality and migration, and show the changes in the size and age structure of the population within a specific time span. Since demographic processes evolve gradually in time and their full impact on the situation of the population often is not apparent until several decades later, official population projections frequently cover longer periods of 30 to 50 years. Thus, it is possible to disclose as yet invisible but pre-programmed future changes and to deliver important early indicators to politicians, the economy and society. The Federal Statistical Office has carried out population projections since the early 1950s. Since the 1960s, the Federal Statistical Office and the statistical offices of the Länder have co-ordinated the assumptions to be made at the national and Länder levels - including those on domestic migration - and the methodological questions arising from these "co-ordinated population projections". The results of these projections are consistent for the Federation and the Länder. Apart from that, the Federal Statistical Office collaborates on population projections for the Federal Ministry of the Interior.
How is a population projection conducted?
A population projection is carried out using a nationally and internationally proven and commonly applied method. At first, assumptions are made concerning the development of fertility, life expectancy and migration up to the target year. These assumptions are based on retrospective studies of the development of the above-mentioned components and of the factors determining them in Germany as well as in countries with comparable demographic trends, and on hypotheses on the social and political changes that, from today's point of view, are likely to occur. Frequently, differing assumptions are made on the trends of the individual components, which, in combination, offer several scenarios of population projections. The projection starts from the actual population in a breakdown by age and sex at a specific reference date and is then carried on for each year of the projection period. The living cohorts are taken forward to the next higher age levels by deducting the expected number of deaths and taking into account the relevant net migration. At the same time, the live births composing the new age groups are added. These new age groups are also adjusted year by year for the expected rates of mortality and migration. The number of live births and deaths is calculated by applying the age-specific fertility and mortality assumptions to the current population. Migration is projected on the basis of the assumed balance of migration and its age structure.
How accurate are population projections?
Population projections are based on hypotheses and therefore are subject to uncertainties. Their results depend, on the one hand, on the current number and structure of the population and, on the other, on the assumptions regarding fertility, life expectancy and migration. As the increasing distance from the base date makes it more and more difficult to predict the components' development, long-term population projections have a model character. Calculating several scenarios makes it possible to illustrate the scope of potential change resulting from alternative assumptions on the development of the determinant components. When unforeseeable events occur, such as epidemics, wars or natural disasters, when there are unexpected changes in the generative behaviour of the population, such as the sudden drop in birth rates started by the pill in the mid-1960s, or when migration changes drastically, not even the calculation of alternative variants can increase the accuracy of population projections. The ultimate purpose of population projections, however, is not to exactly predict future developments. They are rather intended to show how the population size and structure might change under specific conditions.
