Operational Research, Econometrics, and Software Engineering 1 Keep it Descriptive Stupid!
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KindredKIDS:KeepingitDescriptivein
OperationalResearch,Econometrics,and
SoftwareEngineering
MatthijsdenBesten
23rdNovember2004
1KeepitDescriptiveStupid!
Multi–agentbasedsimulation(MABS)isbecomingincreasinglydescriptive.According
toEdmondsandMoss(2004,§2)thisisonlynaturalasMABS“allowsandfacilitates
amoredirectcorrespondencebetweenwhatisobservedandwhatismodelled.”But
shouldMABS’naturaltendenciesbecheeredorbooed?
ProponentsofMABSmightarguethatthehighlevelofdescriptivenessinMABS
reflectsthevirtuousfeaturesoftheapproach.Butwhatifthehighlevelofdescriptive-
nessandcorrespondinglackofsimplicityisratherasymptomofflawedandamateurish
research?Alsomodelsthatarenotbasedonmultipleagentscanbemoreorlessdescrip-
tive.Infieldsasfarapartasoperationalresearch,econometrics,andsoftwareengineering
agreatdealhasbeenwrittenonmodelconstructionandspecificationanditwouldbe
interestingtoseewhetherthisliteraturewouldratherplaceacolonoracommabetween
“keepitdescriptive”and“stupid”.
2KIDSinOperationalResearch
“Keepthemodelsimple”isstilltheoverarchingadviceinoperationalresearch(Robin-
son2004,§5.4.2).Despitethis,alsoinoperationalresearchthesizeandcomplexityof
modelsisgrowingmoreandmore.Chwif,BarrettoandPaul(2000,§3)identifyseveral
technicalandnon-technicalreasonsforthisdevelopment.Onthenon-technicalside,
thereisa“showoff”factorinthatmanagersmightbemoreimpressedwithcomplex
modelswithagreatdealofanimation;thereisan“includeall”syndromeinthatinex-
periencedmodellersmightfeelinsecureaboutexcludinganythingfromthesimulation
model;andthereisa“possibility”factorinthatsoftwaretoolsandcomputermakethat
complexityandsizedonotconstraintmodelbuildinganymore.Onthetechnicalside,
thereisalackofunderstandingoftherealsystem;thereisaninabilitytomodelthe
problemcorrectly;thereisaninabilitytotranslateorcodetheconceptualmodelinto
1acomputerisedmodel;orthereislackofclearsimulationobjectivesthatmightinhibit
theconstructionofsimplemodels.Onthewhole,theoperationalresearchcommunity
recognisesthatover–simplifiedmodelsmightbelessvalid,morelimitedinscope,and
moredifficulttounderstandthanmoredescriptiveanddetailedmodels,butthisisre-
gardedasasmallpricetopaysincesimplemodelsareeasiertoimplement,validate,
andanalyse,areeasiertochange,andrequirelesstimetoinvestigate(Chwifetal.
2000,§5).What’smorethecommunity’sverdictisbackedupbyextensiveexperience
(RobinsonandPidd1998,Tilanus1985).Constructivelysimplemodels“encouragean
evolutionaryapproach;encouragethedevelopmentofaseriesofsmallmodels;reduce
theneedforon-goingassistance;aremorecloselytailoredtoclientcapabilities;facilitate
obtainingmanagementsupport;facilitateuser-participation;[and]facilitatesellingthe
system”(Ward1989).Yet,sometimesmoredetailanddescriptioncanbegoodwhenthe
sacrificeofsimplicitymakesthemodelmoretransparenttotheclientsandparticipants
(Ward1989).
3KIDSinEconometrics
Itusedtobethecasethateconometricianscontendedthemselveswithestimatingthe
parametersofamodelprovidedbyeconomictheory,butsincethemid-1970sadebate
hasbeenragingabouthowmodelsshouldbespecified.Thedebatespurnedanumber
ofgeneralprinciples(Kennedy2000,§5.3):
•Theoryshouldbethefoundationandguidingforceinaspecificationsearch;
•“Testingdown”ismoresuitablethan“testingup”;oneshouldbeginwithagen-
eral,unrestrictedmodelandthensystematicallysimplifyitinlightofthesample
evidence;
•Modelsshouldroutinelybeexposedtoabatteryofmisspecificationdiagnostictests
beforebeingaccepted;
•Researchersshouldbeobligedtoshowtotheirmodelencompassesrivalmodels;
•Boundsontherangeofresultscorrespondingtodifferentreasonablespecifications
shouldbereported,ratherthanjustprovidingtheresultsofthespecificationeven-
tuallyadopted,andthepathtakentotheselectionofthisspecificationshouldbe
fullyreported.
Althoughthesegeneralprinciplesapplyspecificallytoeconometricswhereitisrelatively
easytogeneratealternativemodelsthatarenotdifficulttovalidate,onecouldseehow
similarprinciplescouldguideMABSaswell.Andtheprinciplesaremorecongruent
withkeepingitdescriptivethanwithkeepingitsimple.
24KIDSinSoftwareEngineering
EdmondsandMoss(2004,§1)acknowledgethat“‘KeepitSimpleStupid”makesgood
senseifyouaredesigningorconstructingsomething’.Nonetheless,eveninsoftware
engineering,thereisatendencytovaluedescriptivenessoversimplicity.Thistendency
isalsoknownas“featurecreep”.Thatis,theproblemsthatsoftwaredevelopersfaceare
notfundamentallydifferentfromtheproblemsfacedbymodellersandhencemodellers
mightbenefitfromtoolsthathavebeendevelopedtoaidsoftwaredevelopment.Morein
particular,theunifiedmodellinglanguagemightbeemployedtomakethemodeltrans-
parenttoclientsandparticipants,versioncontrolsystemsmightbeemployedtokeep
trackofthemodeldevelopment,andsoftwareengineeringmetricsmightbeemployedto
keeptrackofthemodelbuildingexpenditure.
5KeepitDescriptiveStupid?