Page 186 - Planning and Design of Airports

For ecasting for Airport Planning 153

Time series analysis essentially involves extrapolating or project-
ing existing historical activity data into the future. Market share fore-
casting is a simple top-down approach, where current activity at an
airport is calculated as a share of some other more aggregate measure
for which a forecast has been made (typically a regional, state, or
national forecast of aviation activity). Econometric modeling is a
multistep process in which a casual relationship is established
between a dependent variable (the item to be forecast) and a set of
independent variables that influence the demand for air travel. Once
the relationship is established, forecasts of independent variables are
input to determine a forecast of the dependent variable. These tech-
niques can also be referred to as a bottom-up forecast. Simulation
models are often used when one needs very detailed estimates of air-
craft, passengers, or vehicles. These models impose precise rules that
govern how passengers, aircraft, or vehicles are routed, and then
aggregates the results so that planners can assess the needs of the
network or a component of the airport to handle the estimated
demand. Typically the outputs from the other forecasting methods
are used as inputs to simulation models. Forecasts from simulation
models represent snapshots of how a given amount of traffic flows
across a network or through an airport, rather than a monthly or
annual estimate of total traffic.
An important element which should be utilized in any forecast-
ing technique is the use of professional judgment. A forecast prepared
through the use of mathematical relationships must ultimately with-
stand the test of rationality. Frequently a group of professionals
knowledgeable about aviation and the factors influencing aviation
trends are assembled to examine forecasts from several different
sources, and composite forecasts are prepared in accordance with the
information in these sources and the collective judgment of the group.
In some cases, judgment becomes the principal approach used with
or without an evaluation of economic and other factors that are
believed to affect aviation activity. A common approach being uti-
lized more often today for preparing forecasts by judgment is known
as the Delphi method. In this method a panel of experts on a particu-
lar subject matter is asked to rate or otherwise prioritize a series of
questions or projections through a survey technique. The results of
the survey are then distributed to the members of the panel and an
opportunity is provided for each member to reevaluate the original
rating based upon the collective ratings of the group. The reevalua-
tion process is often sent through several iterations in order to arrive
at a better result. In the Delphi method, the results of the technique do
not have to represent a consensus of the panel and, in fact, it is often
quite useful to have a forecast which indicates the spread of the panel
in reaching conclusions on a particular issue.
The preparation of judgmental forecasts which reflect the collec-
tive wisdom of a broad range of professionals has proven to be very