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COHORT-COMPONENT
TECHNIQUE
"Perhaps no
single factor is more important for local government
planning than the size and composition
of a region's population and the way it will change in
the future. Even though the total population may remain
constant, changes in its composition can fundamentally
alter the need for public facilities and services."
(Klosterman, p. 51, emphasis added)
In the previous
section, extrapolation
techniques were utilized to
project total population figures for a period of years
into the future. These figures provide an area with a
general idea of what the future likely holds for that
region. These single figures give an idea as to expected
impacts on infrastructure, programs, and other government
related services.
However, a great deal
of important information is not provided by these
extrapolation techniques. Specifically, a single,
aggregated population figure can provide no insight into
the age and gender breakdown of the projected population.
Will the local populace have more elderly persons? More
children? More young persons who are likely to have more
children? The composition of the local population,
particularly the age breakdown, is immensely important to
local planning efforts. More children usually mean a need
for more schools and other related services. A larger
elderly population points to a different set of service
needs. A growing young adult population also has
tremendous impacts as these age groups drive more, have
children, and generally consume certain services at a
greater rate than other age groups.
Clearly, then, to
obtain more precise and more detailed information about a
current and future population, the inclusion of age in
our projection techniques is a necessary addition. In
addition, these more advanced techniques also usually
include gender in the equation because males and females
experience different mortality rates and only one of
these two groups, females is "at risk" to have
children.
The demand for more
informative and more in-depth projections has led to the
development of the cohort-component projection technique;
a technique that incorporates information about
age/gender cohorts and the different ways a local
population can change in the projection of future
populations.
As spelled out above,
the distribution of the local population by age is very
important to local planning efforts. The incorporation of
age in the model is achieved through the division of a
population into age cohorts. This technique divides the local
population into five year age cohorts, usually by gender
as well. For example, Washington State's 1990 population
by five year age cohorts would be the following:
Washington
State Population, 1990
|
|
Age Cohort
|
Females
|
Males
|
1-4
|
182,642
|
191,715
|
5-9
|
180,763
|
189,877
|
10-14
|
163,708
|
173,093
|
15-19
|
157,448
|
166,401
|
20-24
|
170,701
|
181,829
|
25-29
|
203,072
|
209,385
|
30-34
|
220,717
|
222,345
|
35-39
|
213,188
|
213,478
|
40-44
|
186,478
|
187,771
|
45-49
|
140,033
|
143,292
|
50-54
|
107,298
|
109,083
|
55-59
|
96,912
|
94,683
|
60-64
|
99,065
|
90,311
|
65-69
|
100,216
|
85,773
|
70-74
|
81,815
|
66,826
|
75-79
|
65,207
|
46,396
|
80-84
|
44,122
|
25,622
|
85+
|
39,560
|
15,867
|
Total
|
2,452,945
|
2,413,747
|
The local
population is broken into these cohorts because these age
groups experience mortality at different rates (being
high in the first year of life and high at the older
ages). Also, the sexes experience mortality at different
rates as well, with males generally having higher
mortality rates throughout life. The local population is
often broken down by race as well (for example the above
age/gender distributions for whites, blacks, and Asians)
because minority groups generally have higher mortality
rates than whites.
These age/gender (and
occasionally race) cohorts are usually represented in a population
pyramid, a graphical means
of depicting these local population. For an example of
the above data in a population pyramid, go here.
There are three (and
only three) ways that a local population can change in
size and composition. Each of these three components is
built into the Cohort-Component Model. These are:
1. Births:
The addition of people to a local population can
occur most clearly through the birth of children. The
way this is built into the model is important,
because it illustrates another key concept in
demographics. Only a certain portion of the
population can give birth to children (despite all
the advances of medical science). In demographic
terms, this group of people is deemed "at
risk" for birth.
In demographics, females between the age of 10-49 are
identified as the at risk group. The US Census Bureau
provides race specific fertility
rates for the age
cohorts 10-14, 15-19, 20-24, 25-29, 30-34, 34-39,
40-44, 44-49. (See Klosterman Chapter 6, pp. 81-88)
2. Deaths:
The removal of people from a local population through
mortality. Unlike births, all persons are "at
risk" for mortality. For this reason, mortality
rates are calculated
for all age/sex cohorts. Mortality rates are
relatively high for the first year of life and
generally decrease until the middle ages. Mortality
rates are by far their highest for elderly age groups
(ages 65 and up). However, the Cohort-Component model
actually uses the corollary to mortality rates,
called survival rates. Survival rates provide the likelihood
that a person with a given set of attributes will
survive a given period of time. (See Klosterman
Chapter 5, pp. 65-80)
3. Migration:
The last factor impacting local population size is migration. Migration can actually occur in two
directions:
1) in-migration:
movement into the local area by persons or
families
2) out-migration: movement out of the
local area by persons of families.
Unlike births and
deaths, the migration component does not rely upon
the calculation of a rate, like fertility and
mortality/survival rates. Determining who is "at
risk" for migration is very difficult,
especially for in-migrants, so the migration
component instead relies upon past levels of
migration and then uses these past figures to project
future migration levels. Any loss or increase in
population in a past time period not accounted for by
fertility and mortality rates is assumed to be the
result of migration.
For example, if
between 1980 and 1990 the number of births was 1,000
and the number of deaths was 800 and the total
population changed by 400 persons, then those
additional 200 persons would be assumed to be the
result of in-migration. (See Klosterman Chapter 7,
pp. 89-99)
Example of the Technique
UNDER CONSTRUCTION!!!
The major
additions to the model; age/gender cohorts and
cohort-specific information for the components of
population change births, deaths, and migration.
The
presentation and interpretation of population
pyramids.
The
components of population change: Births, Deaths,
Migration.
The
calculation and use of fertility and
mortality/survival rates.
The
non-use of rates when the migration component is
addressed.
"The
cohort-component technique is widely used because
it provides detailed information on an area's
future population, births, deaths, and migrants
by age, sex, and race. This information is useful
for many areas of planning and public
administration." (Klosterman, p. 107)
The use of a
more complex model to provide projections that
are more specific than just a single summary
figure. Planners regularly use models or
techniques that balance the advantage of more
information with the disadvantage of greater
complexity.
The breakdown
of very complex concepts into component parts. In
this example we have broken down local population
change into three simple pieces: births, deaths,
and migration. Local population change is
certainly affected by an infinite number of
factors that are effectively excluded by this
model. They are outside the model. Planners
regularly do this with other topics: for example,
transportation planners understand trips in terms
of origins and destinations and economic
developers understand the local economy in terms
of basic and non-basic employment.
What
advantages does the cohort-component technique
offer to demographers?
What
disadvantages are inherent to this technique?
What are the
data requirements of the cohort-component
technique? Where is this data to be found?
Are there any
assumptions inherent to this technique? What are
they/ Why might they be problematic?
How might an
analyst generate multiple projections using this
technique? What factors can be manipulated within
this model?
Irwin, Richard. 1997.
Guide for Local Area Population Projections. Bureau
of the Census Technical Paper 39. Washington, D.C.:
U.S. Government Printing Office.
Klosterman, Richard
E. 1990. Community and Analysis Planning Techniques.
Rowmand and Littlefield Publishers, Inc. Savage,
Maryland. See chapter 4-8.
Klosterman, Richard
E., Richard K. Brail, and Earl G. Bossard. 1993. Spreadsheet
Models for Urban and Regional Analysis.
Pittenger, Donald.
1976. Projecting State and Local Populations.
Cambridge, MA: Ballinger.
Pittenger, Donald.
1977. "Population Forecasting Standards: Some
Considerations Concerning Their Necessity and Content."
Demography 14: 363-368.
Pittenger, Donald.
1980. "Some Problems in Forecasting Populations for
Government Planning Purposes." The American
Statistician 34: 135-139.
US Census Bureau
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