What Is Demographic
Oh, do not ask, ‘What is it?'
Let us go and make our visit.
is the study of population change in human societies, of the life cycle
events that generate this change, and of the various factors and
circumstances that influence these events.
long do we live? How many children do we have? How do demographic
conditions today compare with those of our ancestors? How does our
society compare with the rest of the world? What explains differences
between societies and change over time? How do life span and family size
influence population growth? How fast is the population of the world
growing? What are the prospects for future growth? Why are human
populations growing older and what are the social and economic
consequences of this aging?
analysis is concerned with the best way to frame and answer such
questions. It is the "how to" of demography. Suppose we wish to answer
the question "How long did people live in 19th century America?" What
sort of facts must be at our disposal? What sort of answers do we
expect? How do we get from the facts to the answer? If we want to know
further how length of life in America today compares with life in the
19th century, how do we proceed? How then to explain the difference?
And, perhaps, generate a prognosis for length of life in the 21st
century? Demographic analysis does not tell us everything we need to
know to answer all such questions, but it tells us many things we must
know to answer them successfully.
information takes many forms, from fossilized remains to birth and death
dates on tomb stones to modern censuses, surveys and vital and
population registration systems. The accomplished demographer learns how
to make the most of any source and how to integrate information from all
available sources. The key to doing this is to learn population concepts
in their pure form, rather than in the manifestations they take when
confronted with particular sources of information. One may then be as
comfortable dealing with the historian's documentary sources or the
anthropologist's field notes as with the modern censuses and surveys
that are the demographers stock in trade.
with any discipline, the substance of demographic analysis lies in
details that can be appreciated only as they are mastered, one step at a
time. There are however three broad themes that pervade and inform the
The raw materials of demography are records of the persons who comprise
particular populations, records that tell of the various demographic
events these persons experienced and the various characteristics that
described them at different times in their lives. When assembling this
raw material, we are concerned with particular persons. Demography is
not about persons as such, however, but about populations and about
persons as members of a population. Populations have an existence and
identity in time beyond the existence and identity of their members.
Persons are born, live out their lives, and die. Populations endure,
often for scores or hundreds of generations.
focuses in the first instance on populations of persons comprising some
human society, but we quickly learn to think more generally of
populations as groups of persons or other entities whose membership
changes with time. In studies of marriage and the family we consider
populations of married couples, populations of single parents, and
populations of families with a given number of children. The ideas we
learn for the purpose of studying how long people live apply equally
well to studying the ages and which men and women marry, how long
marriages last, and how long couples wait before having a first or
another birth. This generality is perhaps the most important single
concept of modern demographic analysis.
to Risk Demography studies numbers of births, deaths and other
events, but these numbers alone rarely get us very far. That there are
more deaths in China than in Monaco says nothing of the comparative
mortality conditions in the two countries: there are more deaths in
China because there are more people in China. To compare mortality
conditions we must relate numbers of deaths to the numbers of persons
‘at risk' of death. More generally, the number of occurrences of an
event of any given class should be related both to the number of persons
who might have experienced that event and to the length of time over
which they might have experience the event. This is the principle of
exposure to risk.
and Cohort Different demographic questions point to different ways
of looking at demographic data. To study population growth we organize
data by time periods. How big was the population at the beginning of a
given period? How many persons were added during the period? What was
the balance between birth, death and migration? Answers to such
questions are period statistics. The most familiar demographic
indicators, such as the population growth rate and crude birth and death
rates, are period statistics.
we are interested in how long people live, or in how many children
families have, we will prefer in the first instance to work with life
histories, records of individuals showing time of birth and death and
other information. Given a suitable assemblage of life histories, it is
a simple matter to calculate average length of life for all persons who
are born during some time period or average number of children born to
women who themselves were born or marry during some time period. These
are examples of cohort statistics, so called because they refer
to the experience of a group of persons defined by having experienced a
particular event during a particular time period.
and cohort statistics are both natural and important ways of looking at
demographic phenomena. Some questions call for period statistics, other
questions for cohort statistics, and still other questions for
understanding the relationship between them. The study of population
growth must begin with period statistics. The question "How rapidly is
the population growing?" cannot sensibly be posed in cohort terms. On
the other hand, it makes no sense to ask "How long did people live in
1980?" We live out our lives over many decades. Yet there ought to be
some way to say how long we live "these days" and how this compares to
how long people lived in the 19th (or the 10th) century. Such questions
require understanding relationships between period and cohort
Griffith Feeney is a demographer specializing in demographic
measurement and analysis. He received a Ph.D. in demography from the
University of California at Berkeley in 1972 and is currently a Senior
Fellow at The East-West Center in Honolulu. He may be contacted by email
firstname.lastname@example.org, by mail at East-West Center, 1601 East-West
Road, Honolulu, Hawaii 96848, or by telephone at 1-808-944-7456. You may
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© 1998 by Griffith Feeney.