beginning at the turn of the century, urban and regional
planning has been largely justified as an institutional
mechanism for providing information about the future to
guide current decision
making." (Klosterman, p. 3)
At all levels of
geography and at all levels of government, one of the
primary tasks for professional planners is to provide
some guidance for decisions that are likely to have a
great effect upon the future of a given area. This
guidance often takes the form of determining existing
conditions, calculating recent changes that resulted in
those current conditions, and then projecting those
changes into the future. In other instances, a planner is
called upon to use their expertise of local conditions
and trends to forecast future conditions. In other cases
where little data is available and local conditions are
rapidly changing (like emerging suburban areas), the
planner must rely upon intuition, opinion, and a little
luck to provide accurate forecasts for the future.
In short, the very
term "planning" suggests that the profession is
intimately involved in the task of foretelling the
future, usually based upon intuition, personal opinion,
or, hopefully, through the use of rational, defensible
methods. Although sometimes haphazardly applied, a core
set of demographic projection methods have been developed
for use by planners in small area and large area when
developing forecasts. It is to these methods that we will
turn our attention to shortly.
responsibilities of the profession, the estimation of
current levels and the prediction of future levels of
population remain among the most important for planners.
At any level of geography, the full range of revenues,
services, conditions, and needs are predicated on the
local population level. Clearly, "everything flows
from population". For example:
revenues are generally tied to the number of
persons (or households). A declining population
is leads to declining revenues for a
municipality, hence the importance of the issue
of "growth" to local governments.
services are intimately tied to population levels
as police offices, fire stations, park acreage,
sewer capacity, and a host of other services
directly linked to the size of the local
population, usually in the form of local
standards for these services (i.e. 10 acres of
park land per 1000 people).
infrastructure is impacted directly by the number
of people in the area with larger numbers leading
to more cars, more pedestrians, more bus riders.
Prior introducing the
demographic analysis/projection techniques, a word or two
about vocabulary. Like any technique, there is a certain
level of jargon associated with these techniques.
Specifically, we need to distinguish between estimates, projections,
An indirect measure of a past or present condition that
can be directly measured. For example, you can estimate
current ridership levels on a bus route by counting
riders on a sample of days during a given month.
planners do not make "estimates" of future
conditions. An estimate is used only for past or current
conditions and therefore does not require anything in the
way of a "crystal ball". Also, unlike
projections and forecasts, estimates are directly
measured and therefore are based upon what is known
and able to be checked for accuracy.
A conditional "if, then" statement about the
future. For example, if a city grew by 10% between 1980
and 1990 from 10,000 to 11,000, then given a continued
ten year growth rate of 10%, that city will have a
population of 12,100 in the year 2000.
A projection is the
most popular and most defensible "crystal ball"
method because it is impossible to make an incorrect
projection if one's calculations are correct. A
projection simply states that "if the following
happens, then the future will be this way". If the
following does not happen, the analyst has not made a
mistake, rather the conditional if, then statement did
A judgmental statement that is the "best guess"
about a future condition; a forecast is the analyst's
determination of the most likely future. For example,
given a range of projections, an analyst may decide that
because of the strong economy and growing popularity of
the Northwest, King County's population is expected to
grow by 30% between 1990 and 2010. This would be that
analyst's best guess about the future, not a conditional
statement about future conditions.
Because of their
difficulty, high likelihood of inaccuracy, and
controversial results, forecasts are often left to
well-paid consultants that have mastered the various
projection methods. Population forecasts are usually a
necessary, but inherently incorrect part of the planning
process. At some point a "most likely" future
must be determined and appropriate actions must then
follow from this determination.
However, only very
rarely are forecasts very accurate, and more often than
not unforeseen (and oftentimes unforeseeable) events
result in wildly inaccurate forecasts (just look at King
County's population projections of the 1970s for an
excellent example of this). When possible, then, planners
should rely upon projections, not forecasts, to inform
and advise decision-makers.
There are two common
techniques utilized by planners that are projecting
future conditions. Each has its own advantages and
disadvantages and should be used only with careful
consideration of their limitations.
techniques use total population figures from the past
to project future population levels. These techniques
do not disaggregate the population figures and
therefore are easy to calculate, but limited in their
technique is more data intensive because it
disaggregates total population figures into
age/gender (and sometimes race) "cohorts".
Further, the different "components" of population change (births,
deaths, and migration) are taken into account and
past figures for these components are applied to the
current age/sex cohorts.
When undertaking this
all important task of projecting future conditions, a
quote from Klosterman should guide your work:
planners' task is to combine the most reliable
information about the past with the most appropriate
assumptions about the future to prepare the best
possible forecasts." (Klosterman, p. 4)
importance of population projections and their
implications for planning at all levels of
government and all levels of geography.
difference between estimates, projections and
fundamental differences between the Extrapolation
Technique and the Cohort-Component Technique.
As the name
"planning" suggests, one of the major
purposes of the profession is to help an area get
ready for future conditions (to plan!). However,
looking into the crystal ball is a difficult and
hazardous task as projections are limited by the
likelihood of unforeseen events.
projections, it is important to develop a range
of future conditions to illustrate that the
future is largely an unknown quantity. In doing
so, even when making a forecast, an analyst can
provide an ample "range" within which
planning can take place.
Armstrong, J. Scott.
1978. Long-range Forecasting: From crystal Ball to
Computer. New York: Jon Wiley and Sons.
E. 1990. Community and Analysis Planning Techniques.
Rowmand and Littlefield Publishers, Inc. Savage,
Maryland. See Chapters 1-8.
E., Richard K. Brail, and Earl G. Bossard. 1993. Spreadsheet
Models for Urban and Regional Analysis.
1976. Projecting State and Local Populations.
Cambridge, MA: Ballinger.