Encyclopedia of Sociology

DEMOGRAPHIC METHODS

We might note, for example, that 20 percent
of Norway’s population in 1997 was under the age
of fifteen. In contrast, 39 percent of Mexico’s
population seven years earlier fell into this catego-
ry. But it is perhaps more dramatic to create a
visual display of these figures in the form of age
pyramids.

An age pyramid is typically constructed as a
bar graph, with horizontal bars—one representing
each sex—emanating in both directions from a
central vertical age axis. Age increases as one
proceeds up the axis and the unit of the horizontal
axis is either the proportion of the total popula-
tion in each age group or the population size itself.

We see in panel A of figure 1 that Mexico’s
population is described by a very broad-based
pyramid—actually, in two-dimensional space, a
triangle, but by convention we refer to it as a
pyramid—which reveals a remarkably large pro-
portion of the population not yet having reached
adulthood. The median age of this population is
about twenty years. Such a distributional shape is
common particularly among high fertility popula-
tions. In stark contrast we have the pyramid shown
in panel B, representing the population of Nor-
way. Rather than triangular, its age distribution is
somewhat more rectangular in shape, which is
typically seen among countries that have experi-
enced an extended period of low fertility rates. It is
easy to see that Norway’s median age (about thirty-
six years) is considerably higher than that of Mexico.

As mentioned above, not only can we examine
the age structure of populations through the use
of pyramids, but we can also gain much insight
into a nation’s history insofar as that history has
either directly or indirectly influenced the size of
successive birth cohorts. Note, for example, the
age pyramid reflecting the population age struc-
ture of France on 1 January 1962 (figure 2).

In this figure, several notable events in France’s
history are apparent. We see (1) the military losses
experienced in World War I by male birth cohorts
of the mid to late 1890s, (2) the remarkable birth
dearth brought about by that war (the cohort aged
in their mid-forties or so in 1962), followed by (3)
the return of prewar childbearing activity once the

war ended (the cohort who in 1962 were in their

early forties), and a similar pattern revolving around

World War II, during which (4) a substantial de-

cline in births took place (the cohort around age

twenty in 1962), succeeded by (5) a dramatic baby

boom in the years immediately following (those

around age fifteen in 1962).

COMPARISON OF CRUDE RATES

Much of the work in which social scientists are

engaged is comparative in nature. For example, we

might seek to contrast the mortality levels of the

populations of two countries. Surely, if the two

countries in question have accurate vital registra-

tion and census data, this task would appear to be

trivial. For each country, we would simply divide

the total number of deaths (D) in a given year by

the total population (P) at the midpoint of that

year. Thus we would define the crude death rate

(CDR) for country A as:

DA [t,t+1] [nMx

A. nPxA]

PA [t+.5]

CDRA = =

∑

ω-n

x= 0 ,n

∑

ω-n

x= 0 ,n

nPxA

( 1 )

where t denotes the beginning of the calendar

year, ω is the oldest age attained in the population,

nMx

A is the death rate of individuals aged x to x + n,

and nPxA is the number of individuals in that same

age group. nMxA and nPxA are centered on the mid-

point of the calendar year. The rightmost segment

of this equation reminds us that the crude death

rate is but the sum of the age-specific death rates

weighted by the proportion of the population at

each age.

Unfortunately, comparisons of crude death

rates across populations can lead to misleading

conclusions. This problem is in fact general to any

sort of comparison based on crude rates that do

not account for the confounding effects of factors

that differentiate the two populations.

Let us examine the death rates of two very

different countries, Mexico and Norway. The crude

death rate (for both sexes combined) in Mexico in

1990, 5.2 per 1000, was approximately half that for