4. Problem perspectives
In a broad sense, there are two distinct classes of
LRP studies with regard to their problem perspectives.
One of these classes is a single-stage LRP
which is primarily concerned with both the location
of facilities serving customers and the establishment of outbound delivery routes around those facilities
(see Fig. 2). A no th er class is a two-stage LRP which
expands its problem scope to consider two-layers of
the production-distribution network involving both
outbound (delivery) and inbound (pickup) distribution
processes. These classes can be further subdivided
into two types of models: deterministic and
stochastic. The deterministic LRP model assumes
that the nature of location/routing parameters such
as demand/supply size is known and fixed with
certainty, whereas the stochastic counterpart consider
an uncertain and random nature of those parameters.
Within each of these broader categories, there are
nine subcategories that reflect many practical aspects
of LRP (see Table l(a)>. These subcategories include
two layers of distribution facilities: primary and secondary.
A primary facility represents either the origin
or destination of a vehicle journey. Some examples
of primary facilities are manufacturing plants,
hospitals, waste collection centers, airports, and
landfills. On the other hand, a secondary facility
represents an intermediate or a transshipment point
such as a military depot, a warehouse, a distribution
center, a waste transfer station, a consolidation terminal,
and a break-bulk terminal. Many LRP studies
which were concerned with the location of secondary
facilities typically assume that primary facilities are
located at known and fixed sites as shown in Fig. 3.
The main intent of these classification schemes is to help academicians and practitioners alike to correctly
identify the specific scope of the LRPs they encounter
and then encourage them to capture more
realistic aspects of LRPs.