The Dictionary of Human Geography

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points, weighted by their populations and the

distances between them (as in thegravity

model). Population may be replaced by

another variable, such as purchasing power, to

give a measure of marketaccessibilityfor

each pointi. rj

Suggested reading

Stewart and Warntz (1959); Warntz (1965).

population projection A scenario about the

size and composition of a population in the

future (or past) that is based on a set of demo-

graphic assumptions. As ‘what if’ scenarios,

projectionsshould be distinguished from esti-

mates (which aim to provide the most accurate

picture of current populations) andforecasts

(which aim to statistically predict the most

probable scenario based on a broad range of

information). The key (direct) mathematical

methods for making projections include the

extrapolation of demographic trends, the use

of the balancing equation (seedemography)

and, notably, the cohort component method,

which makes specific assumptions about the

fertility, mortality and migration

experiences of every five-yearcohortin a

population as it ages. The increased applica-

tion of population projections in business and

planning has helped fuel a virtuous circle of

methodological innovation, which includes

developments in sub-national projections,

small-area projections, multi-region models

and ethnic minority projections (Wilson and

Rees, 2005). ajb

Suggested reading

Rowland (2003, ch. 12).

population pyramid A graphical depiction

of the age–sex structure of a population.

This provides a visual record of the current

population structure and differentiates

between expanding populations (pyramid

shape with concave sides), stationary popula-

tions (straight-sided) and constricting popu-

lations (beehive shape with convex sides: see

the US Census Bureau for contemporary pyr-

amids that can be animated). Pyramids also

provide visual clues as to prior demographic

events, including the arrival/departure of sex-

selectivemigration streams, the impact of

1897

1907

1917

1927

1937

1947

1957

1967

1977

1987

1997

Year of birth

1897

1907

1917

1927

1937

1947

1957

1967

1977

1987

1997

Year of birth

Age

100

90

80

70

60

50

40

30

20

10

0

500 400 300 200 100 0 0 100 200 300 400 500

Size of age groups (thousands) Size of age groups (thousands)

MALES FEMALES

1

2

3

4

5

1

2

3

4

5

1 Shortage in births due to the 1914–18 war (‘empty classes’)

2 Passage of ‘empty classes’ to age of fertility

3 Shortfall in births due to 1939–45 war

4 Baby boom

5 Non-replacement of generations

population pyramid France, 1897–1997(Le ́vy, 1998, p. 4)

Gregory / The Dictionary of Human Geography 9781405132879_4_P Final Proof page 555 1.4.2009 3:20pm

POPULATION PYRAMID