Samuel Clark Department of Sociology, University of Washington Institute of Behavioral Science, University of Colorado at Boulder Agincourt Health and Population Unit, University of the Witwatersrand Population Projection

1 Projections & Forecasts  Population projections are most recognizable and requested “demographic output”  Used extensively for planning: – Governments – School districts – Corporations – NGOs – And many more  Main world projections produced by: – Census Bureaus – UN – US Census Bureau

2 Projections  A population projection models the size and composition of a population through time …  Projections used in two senses – Produce useful numbers, and – Examine relationships between demographic variables and implications of changing those relationships; “what if” kinds of questions  Example: – Effects of HIV on mortality, fertility and marriage and how all of these effects work together to alter the size and composition of a population, – How does treating or eliminating HIV change these?

3 Forecasts “Population projections are calculation which show the future development of a population when certain assumptions are made about the future course of fertility, mortality, and migration. They are in general purely formal calculations, developing the implications of the assumptions that are made. A population forecast is a projection in which the assumptions are considered to yield a realistic picture of the probable future development of a population” (UN)

4 Projections & Forecasts  A projection is judged by how well it reflects the internal relationships that exist in a population  A forecast is judged by how well it predicts the future size and composition of a population  Generally better projections also yield better forecasts  Projections can also be made for the past – “backward projections”, and they can start at a point in the past and project forward under different assumptions

5 Projection Methodologies  There are a variety: – Continuous differential equations – Discrete time systems – One-sex / Two-sex – Age structured or not – Many levels of detail/complication

6 Cohort Component Method  The cohort component method (CCM) is the standard method used for most population projections and many forecasts  CCM is: – Age structured – Either one or semi-two –sex – Discrete (population components calculated at certain moments in time with long relatively long intervals between)  CCM projects a population through a set of age groups and time periods of the same width

7 Cohort Component Method  For each projection period, CCM consists of three steps: 1.Project forward the population in each age group to determine how many are still alive in the next period in the next age group, 2.Compute the number of births produced during the projection interval by each age group of women, sum these over all age groups, and survive them to the beginning of the next age group, and 3.Add immigrants and subtract migrants from each age group and repeat steps 1-2 for the immigrants

8 The Two-Sex Problem  Births are produced by a process that involves both sexes: – Pairing – Sex within pairs – Successful pregnancy & live birth  In practice this is very complicated and a (slightly unsatisfying) shortcut is usually applied: – Model an all female population (women and daughters) – Add males back in …  We will now go through the construction of a “female dominant” CCM projection …

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