The birth rate(also called the crude
birth rate) is usually represented in
demography as the ratio of live births in
an area population annually, most often
expressed as the annual number of live
births per 1,000 people (or a time divi-
sion, such as at mid-year). Related to this
is the infant mortality rate,which is the
annual number of deaths of children less
than one year old per 1,000 live births.
The death rate,or crude death rate,
refers to the ratio of deaths in an area
population annually. It is often expressed
as the annual number of deaths per 1,000
people. Life expectancy is related to
death rate, and indicates the average
lifespan of people within a certain popula-
tion. It is calculated on the basis of statis-
tical probabilities, usually represented by
the average number of years a group of
people born in the same year can be
expected to live, if mortality at each age
remains constant in the future.
What is a population pyramid?
A population pyramid is actually two back-to-back bar graphs that show the distribu-
tion of a population by gender: One shows the number of males and the other shows
the number of females (most often in five-year age groupings). These “pyramids” take
a variety of shapes. For example, a triangular population distribution (also called a
pyramid or exponential distribution) is usually indicative of a high birth and death
rate, and a short life expectancy that is typical of less economically developed coun-
tries. A rectangular population distribution pyramid shows little change in size
between the age groups, with more people reaching old age; this is typical of a more
economically developed country.
There are several uses for population pyramids. Besides calculating life expectan-
cies, they are often used to find the number of dependents being supported in a specif-
ic population. (Dependents in this case are considered to be children under 15, those
attending school full time and “unable” to work, and those over 65 years old or
retired, although this is a general definition and not applicable in all cases in all coun-
tries.) Thus, governments can use such graphs to determine how much of the working
population can support dependents. The graphs are also used to help predict future 381
MATH IN THE HUMANITIES
Based on its shape, a population pyramid quickly
shows whether this crowd of people has a high or
low birth rate, as well as a high or low mortality
rate. Lonely Planet Images/Getty Images.