demografie.net
Understanding population dynamics –
What is demography about
Population Dynamics: The Basic Equation
Why populations grow or shrink
Why does one city grow while another shrinks? Behind all these phenomena lies a comparatively simple mathematical logic. For planners and students, it is the indispensable starting point of every demographic analysis.
The size of a population at a given point in time is derived from the population of the previous year, plus births and immigrants, minus deaths and emigrants.
The equation breaks down into two fundamental components: natural population change results from the difference between births and deaths. Net migration is the difference between immigration and emigration – often the more decisive factor for regions within a country.
A particularly intuitive application is the doubling time. As a quick rule of thumb, the rule of 70 applies: the doubling time in years is approximately 70 divided by the annual growth rate in percent. At 3.5%, it is around 20 years.
P(t) = P(t-1)
+ (B − D)
+ (I − E)
Mortality
Measuring mortality and its tools
The simplest starting point is the Crude Death Rate (CDR) – deaths per 1,000 persons of the average population. However, it completely ignores age structure. Sweden has a higher CDR than many Asian countries, not because the risk of dying is greater there, but because the proportion of elderly people is higher.
Anyone wanting to compare mortality rates needs the Age-Specific Death Rate (ASDR), which relates deaths in an age group to the corresponding sub-population.
A special case is Infant Mortality (IMR) – one of the most meaningful indicators of social development. Methodological care is required: depending on the type of analysis, results can differ by up to 50%.
The Life Table
The methodological centrepiece of mortality analysis. From just a few input values, mortality risk, life expectancy and survival probability can be derived.
Period Life Table (PLT)
Cross-sectional analysis of a single year. Fictitious cohort of 100,000 persons. Basis of the well-known life expectancy figures.
Cohort Life Table (CLT)
Tracks a real birth cohort throughout its entire life. Delivers actual, not fictitious, values.
Fertility: More Than Just the Birth Rate
From TFR to the reproduction rate
Whether nurseries are built or closed, whether pension funds remain financially viable – all of this depends directly on fertility.
The best-known indicator is the Total Fertility Rate (TFR): how many children a fictitious woman would bear if she were as fertile in every year of age as the annual average. The TFR is a period measure – quickly available, but susceptible to the tempo effect. If women delay births, the TFR falls without the final number of children changing.
As a rule of thumb: a TFR of around 2.1 is the replacement level for industrialised countries. The generation interval – the average age of mothers at first birth – significantly influences population dynamics, even if the total number of children remains stable.
TFR – Total Fertility Rate
2,1
Replacement level in industrialised countries
Key indicators
Migration: The Underestimated Driver of Population Change
The fourth major parameter of population dynamics
Few demographic topics are discussed so intensively in public debate and simultaneously so frequently misunderstood as migration. For planners and students it is therefore especially important to look beyond the political charge of the subject and understand migration for what it demographically is: the fourth major parameter of population dynamics – alongside births, deaths, and age structure.
The term refers to all permanent or long-term relocations of a person's main place of residence that cross administrative boundaries. Migration is therefore clearly distinct from tourist movements, commuting between home and workplace, or the temporary absence of soldiers, sailors, or prisoners – groups assumed to retain their actual home base. This distinction sounds technical but has considerable practical consequences: anyone planning for a population must know who is actually being counted.
Demography distinguishes fundamentally between internal migration, i.e. movements within a country across municipal boundaries, and international migration, where at least one national border is crossed. Depending on direction, one speaks of immigration (in-movement) or emigration (out-movement). The motives are highly varied: from classic labour migration and the desire for better living conditions, via educational migration, to flight from war, political persecution or ecological disasters.
Push factors
Unemployment, insecurity, lack of prospects, war, persecution
Pull factors
Job opportunities, quality of life, social networks, political stability
How is migration measured?
The most fundamental measure is the migration balance, i.e. the difference between immigrations (I) and emigrations (E) in a given year. A positive balance indicates a net immigration surplus, a negative one a deficit. To compare regions of different sizes, the balance is related to the average population – giving the net migration rate:
R(Mi) = (I(t) – E(t)) / P(t) × 100
Also significant is the migration volume, i.e. the total number of in- and out-movements regardless of the balance. It describes the intensity of migration, regardless of whether the net result is zero or not. The ratio of migration balance to volume gives the migration efficiency – a measure of how strongly the movements actually affect population size on a net basis.
A conceptually important term is the base migration: the level at which in- and out-migration balance each other and the migration balance is consequently zero. Crucial for planners, however, is that even a balanced migration balance can substantially alter a region's age structure – namely when the age structure of in-migrants systematically differs from that of out-migrants. Young people typically move into cities, while somewhat older population groups tend to move to suburban areas.
Migration in practice: The example of Sweden
Few countries illustrate the changeability of migration patterns as impressively as Sweden. Until the late 1920s, Sweden was a classic emigration country – between 1851 and 1929 the migration balance was minus 1.1 million, most of whom emigrated to North America. Since 1930, the country has not recorded a negative migration balance. Since then, on balance, over one million more people have immigrated than emigrated.
Within barely 50 years, Sweden developed into a multi-ethnic society – with all the social opportunities and planning challenges that entails. Today, one in five people permanently residing in Sweden was born abroad.
This example illustrates a core insight that should be central to every planner: migration patterns are not stable. They respond sensitively to economic cycles, political events and global crises – and can reverse fundamentally within a few decades. Anyone planning municipal or regional infrastructure on the basis of today's migration trends, without factoring in their possible volatility, is on thin ice. Demographic analysis of migration provides the necessary tools – not to predict the future, but to realistically assess the range of possible developments.
Projection or Forecast?
What demographic projections really achieve
Takes a position
Starts from a defined initial situation and an assumed development path and links the result to a probability of occurrence.
"If nothing unforeseen happens, the following will occur."
Takes no position
Consistently extrapolates a trend under fully defined assumptions – without a probability statement. It can deliberately lead to absurd results to illustrate the limits of a development.
No field is subject to more public confusion
Anyone who plans, communicates or researches should master this fundamental conceptual dualism. The methods of projection calculation – from linear extrapolation to the cohort-component method – can be explored interactively on the Future Outlook page .