BOOKS OF RtfiDIfGS - PAHO/WHO

- 294 -
thcir validity. Cretin [!7] analyzcd ilhe process of dcathi followinii FACILTY
nmyocardial infarction and developed a model of risk of death fronm LOCATION
inriarction. ¡ler model combined features of both the disease process
a;i.d tie msiedical iolpunse and was used to evaluate policy alternatives.
Thec disease was nimodeled as a discrete Mlarkov process, withi transition
probabilities and other paranimeer: obtaincd froin statistical regression.
I'lhi is an interesting approach, whlici coulhl form 3n integral
l:art ou broa;der rm ,dels.
l1cvcs,,olS aind Willeniain [18,19] vicwed thie scrccnitig process as
a triage system. in which calis were alloated to prinmary service if
Ihey wiece ciecrgcncies and to secondary service if they werc not.
Cadlls wicrle aso routed to tile ecoundary service il all priuniary service
vehicilhs were buisy. lhe analysis consisted of comibining assiarimed decision
and evesst probabilities. Although the approaci was straightl.
forward. tile assurmed prohabilities were not justified by the autihors.
Ti'ese models and ,,ncceptualizations representl tle state of the
;ar as relletied by tihe EMS-oriented literature iup to about oIIc ycar
ago. 1i is clear tisat there is great potential íor lurther application of
locatiota niodels to ENIS problems. We believe such applications will
draw heavily on the cointext.free location litera;iav specifically on
tihe Imaxinmal covering model and its refisenicenis, whllli atre distIissce
below.
Context-free Location Analysis
An interlocking set ol tihree network loiation mnodels will be
described here. A!' :aideb hta;.. have been applied. at least conceptually,
lo emergency seivices. Tite third model (tile ttmaxiinal covering
model), whose lurdier developmient and refincient we sihall discusIs
has recently bcen applied to the location o[ fire stationis and
equipment in Baltimore,'MD. I'lis i¡, to our knowlelge. tíhe first
applications of this model to such a seiting, although it is chiaracterized
by the moht versatile and adaptable objective of all thrce niodels.
The p¡Median Problem
The location model that underlies, tieoretic!"l)y and historically,
the others we sliall discuss is the p-median problemn. whicil was pro-
Iseld by Hakimi [20,21] in the context of locating switchiing centers
in a communications network. It was recognized. lhowever, that the
probiem statement could be extended to many otiher fiames of reference.
We will state it in the setting in which people come to facilities
lor service, but it can easily be turned around to pertlin to the situation
of serven going from facilities to points of demand.
Locate p faciliies on a network ol denreds so tha tihe average travel
lime of all uen is a minimnum. Since fhe Ifailities ate not dstiinguishetl
Lt sire or specialiaiion. it is asumed tihat every user will travel lo hisa
uearest facility. ,tJ MM ER
Numerous researchers iave propxned sulution , .lihods tor thc
p-median prublenm. and solution procedcsílt' are imporirant: a well-