Demography: Population Change and Perception Flashcards
What are the most complete and reliable sources of information on population? Their disadvantages.
The most complete and reliable sources of information on population are the census and sample surveys.
However, these are;
- Expensive,
- Laborious and time consuming, and
- May also contain errors
What are the substitutes for estimation and projection?
- Population estimation and projection are therefore used as substitutes for census and sample surveys in providing data for the present and future dates respectively
– Interpolation = estimation
– Extrapolation = projection
What is population estimation used for? How do populationchanges take place and In which counteries are they conveniently used?
Population estimation is mainly used in estimating present and inter-censal populations
Population changes take place through births, deaths and migration
In countries where data on vital rates (births, deaths and migration) and reliable census counts are available, this method is conveniently used
What is population projection?
- Forecast of population change using estimates of fertility, mortality, and migration
– Projections may extend for varying numbers of years into the future
What are the four main methods used to measure population changes?
- Book keeping equation
– Simplest method to estimate population change, adds the births and in-migrants to the initial population and subtracts the deaths and out-migrants - Linear change
– Assumes population size changes by exactly the same amount during a specified time period - Arithmetic Formula
– If we assume that the average annual growth rate between two censuses is the same - Geometric change
– Assumes population has a geometric growth rate - Exponential change
– Assumes population has an exponential growth rate
Book keeping equation/The component method
The population changes by adding births and in-migrants and subtracting deaths and out-migrants
Pt=P0+(B−D+I−O)
Linear change equation
- Let b = Number added per time unit
b=Pt−P0/ (all over) t - The population size changes by exactly the same amount “b” during each time period “t”Pt=P0+bt
Arithmetic formula
From the general formula for the population growth at a constant rate
Pt = Po(1 + r t) =>Pt = Po + Po r t
Pt – Po =Po r t
r= Pt – Po/
Po t
Define and give the equation for geometric change
The population changes by step, i.e., the increment added (or the decrement is subtracted) periodically
Pt=P0(1+r)t
Where r = geometric rate of growth
Geometric formula for r
r = [(Pt)1/t ] / - 1
(Po)
Exponential change and advantage
The population changes instantaneously and continuously
Pt=P0 e^rt
The advantage of the exponential method is that it’s easy to solve for any parameter. For example;
Exponential growth rate Also,
r=1/t * ln(Pt/P0)
t=[lnPt/P0] /r
What is doubline time? What is the calculation based on?
Time it takes a given population to double in size for a given time period (i.e., time for Pt/P0=2)
Calculation based on exponential growth (“r” fixed in time)
Time is inversely proportional to the population growth rate
Equations for doubling time
Then for doubling time
t=ln(2)/r ,
therefore t is inversely proportional to r
Approximation
t=70/r
What does each variable stand for in the book-keeping equation?
Let Pt = Population at time “t”
Po = Population at an earlier time “0”
B = Births between time “0” and time “t”
D = Deaths between time “0” and “t”
I= In-migration/immigration between time “0” an “t”
O= Out-migration/emigration between time “0” and “t”
