SADC Course in Statistics Population Projections - II (Session 20)

SADC Course in Statistics

Population Projections - II

(Session 20)

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Learning Objectives – this session

At the end of this session, you will be able to

• discuss specimen ways in which population projections can be used in administration

• understand some complexities of forecasting variable components such as migration

• appreciate that projecting components of the PP process can utilise many types of available information

• enjoy exploring more about demographic literature on your own!


Why population projections: 1

Uses of population projections (PPs) become many and various in a society that has the resources with which to plan.

For example:-

National PPs projected school enrolments (in conditions of near-universal attendance)

Local PPs school building programmes.

Consequences of political policy suggestions and decisions can be quantified & costed.



Example: in UK (2007) idea was put out that all children stay in school to age 18 (currently 16). PPs can help with projecting pupil numbers, & therefore space needs, but society also needs to consider nature of teaching (materials, courses etc) to motivate/help those who would have left at 16 if allowed.



Example: in UK (2007) idea put out (by a different political party) that under-achieving primary children should repeat final primary-school year rather than progress to secondary school at 11-12.

Could project effects on school class numbers, space requirements, teacher numbers etc: requires an assumption about no.s who would be affected.


PPs and politics: 1

Above examples refer to possible political decisions with implications for (school) pop.n, and associated resources. Other issues, e.g. migration (graph ) are even more bound up with national-level politics.

A country like UK has an aging pop.n, with increasing percentage aged 60+, while most migrants are in economically-active age groups. This leads to fierce argument (for society, not just for technical demographers) about economic vs. social effects & other pluses and minuses of in-migration.


PPs and politics: 2

The story of migration projections is an interesting one to follow. Plenty material in UK e.g. http://www.migrationwatchuk.org/ and http://www.workpermit.com/ including evidence that the projections shown below are already clearly wrong!

The following graphs were taken fromhttp://www.gad.gov.uk/Population/index.asp?

v=Principal&y=2004&subYear=Continue on the basis stated therein that “the copyright and any other

rights in all material on this site are owned by the GAD” but that “This copyright and other rights do not extend to the GIFs/JPGs, which are publicly available GIFs/JPGs”



Observing smoothness

Usually, change from year to year in probability of dying is small, so complex-seeming prob.s of dying mentioned above quite gradually-changing series, and effects of the changes on projections will only be seen over quite long periods. Note slow steady change of UK e0 values first below. Compare M & F

figures.

The fertility and migration figures are capable of changing considerably faster.




Smoothness: generation vs. period

Note wider variation (vertical scale) of the period TFRs immediately below, vs. the generation-based average completed family-size figures in the next slide below.

In the latter, note there is no single time beyond which figures become projections – there is a long change-over period during which more of the contributing age/period figures are projections for the later-born women.





Steering the projection: 1

The above illustrates how careful analysis – and judgment – of observed series of data values allows components of PP process to be gradually changed over future time ~ for example, figures may be worked out on the basis that experts expect/assume a slow decline in generation fertility rates, combined with increased deferral of first birth to older ages.


Steering the projection: 2

The pattern of shorter-term assumptions can be seen on the website of the Gov.t Actuary’s Department, until recently responsible for population projections.

http://www.gad.gov.uk/population/2004/methodology/fertass.htm http://www.gad.gov.uk/population/2004/uk/wukfert.xls

The current UK longer-term assumptions for the component countries’ ASFRs are graphed below.



OptionsThere is no “single correct” way to develop

no.s to be input to a series of projections over a number of years to come.

For example, at one time:-UK used to assume that over a 40-year

period mortality rates in UK would reduce to equal then-current Swedish values;

Berkshire, UK, steered local area PPs by (i) assuming every new house given planning permission would be occupied – formula for population by house size, and (ii) trends in no. of persons/household would continue linearly downwards


Some practical work follows …

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SADC Course in Statistics Population Projections - II (Session 20)

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