system-dynamic-english

SystemDynamics1CAP4800/5805SystemsSimulationWhatisSystemDynamicsComputersimulationmodelingforstudyingandmanagingcomplexfeedbacksystems,suchasbusinessandothersocialsystemsSystem:Ingeneral,acollectionofinteractingelementsthatfunctiontogetherforsomepurposeHere,feedbackisthedifferentiatingdescriptorPropertiesofdynamicproblemsContainquantitiesthatvaryovertimeVariabilitycanbedescribedcausallyImportantcausalinfluencescanbecontainedwithinaclosedsystemoffeedbackloopsHistoryCybernetics(Wiener,1948):thestudyofhowbiological,engineering,social,andeconomicsystemsarecontrolledandregulatedIndustrialDynamics(Forrester,1961):appliedprinciplesofcyberneticstoindustrialsystemsSystemDynamics:Forrester’sworkhasbeenbroadenedtoincludeothersocialandeconomicsystemsRelyingoncomputer,SystemDynamicsprovidesaframeworkinwhichtoapplytheideaofsystemstheorytosocialandeconomicproblemsSystemDynamicsModelingIdentifyaproblemDevelopadynamichypothesisexplainingthecauseoftheproblemCreateabasicstructureofacausalgraphAugmentthecausalgraphwithmoreinformationConverttheaugmentedcausalgraphtoaSystemDynamicsflowgraphTranslateaSystemDynamicsflowgraphintoDYNAMOprogramsorequationsCriticalAspectsThinkingintermsofcause-and-effectrelationshipsFocusingonthefeedbacklinkagesamongcomponentsofasystemDeterminingtheappropriateboundariesfordefiningwhatistobeincludedwithinasystemUnderstandCause&EffectCausalthinkingisthekeytoorganizingideasinasystemdynamicsstudyInsteadof‘cause’,‘affect’or‘influence’canbeusedtodescribetherelatedcomponentsinthesystemSomearelogical(e.g.physics)FoodintakeweightMoneyhappinessFiresmokeSomearenot(e.g.sociology,economics)UseofseatbeltsreducedhighwayfatalitiesShorteneddaylighthoursincreasedsuicideratesFeedbackThinkingintermsof“causeandeffect”isnotenoughoceanevaporationcloudrainocean…Feedback:aninitialcauseripplesthroughachainofcausationultimatelytore-affectitselfSearchtoidentifyclosed,causalfeedbackloopsisonekeyelementofSystemDynamicsThemostimportantcausalinfluenceswillbeexactlythosethatareenclosedwithinfeedbackloopCausalLoopDiagram(CLD)RepresentthefeedbackstructureofsystemsCaptureThehypothesesaboutthecausesofdynamicsTheimportantfeedbacksCLDExamplesSalaryVSPerformanceSalaryPerformancePerformanceSalarySalaryPerformanceTiredVSSleepTiredsleepSleeptiredTiredSleepAugmentingCLD1(LabelingLinkPolarity)Signing:Adda‘+’ora‘–’signateacharrowheadtoconveymoreinformationA‘+’isusedifthecauseincrease,theeffectincreasesandifthecausedecrease,theeffectdecreasesA‘-’isusedifthecauseincreases,theeffectdecreasesandifthecausedecreases,theeffectincreasesSigningArcsSalaryPerformance+++-TiredSleepAugmentingCLD2(DeterminingLoopPolarity)PositivefeedbackloopsHaveanevennumberof‘–’signsSomequantityincrease,a“snowball”effecttakesoverandthatquantitycontinuestoincreaseThe“snowball”effectcanalsoworkinreverseGeneratebehaviorsofgrowth,amplify,deviation,andreinforceNotation:placesymbolinthecenteroftheloopNegativefeedbackloopsHaveanoddnumberof“–”signsTendtoproduce“stable”,“balance”,“equilibrium”and“goal-seeking”behaviorovertimeNotation:placesymbolinthecenteroftheloop+-CLDwithPositiveFeedbackLoopSalaryPerformance,PerformanceSalarySalaryPerformanceThebetterIperformThemoresalaryIgetThemoresalaryIgetThebetterIperform+++ThemoresalaryIgetThebetterIperformCLDwithNegativeFeedbackLoopTiredSleepThemoretiredIamThemoreIsleepThemoreIsleepThelesstiredIamThelesstiredIamThelessIsleepThelessIsleepThemoretiredIam+--TiredSleep,SleepTiredLoopDominanceTherearesystemswhichhavemorethanonefeedbackloopwithinthemAparticularloopinasystemofmorethanoneloopismostresponsiblefortheoverallbehaviorofthatsystemThedominatingloopmightshiftovertimeWhenafeedbackloopiswithinanother,oneloopmustdominateStableconditionswillexistwhennegativeloopsdominatepositiveloopsCLDwithCombinedFeedbackLoops(PopulationGrowth)BirthratePolulationDeathrate-++++-CLDwithNestedFeedbackLoops(Self-RegulatingBiosphere)SunshineEvaporationAmountofwateronearthRainCloudsEarth’stemperature-+-++++++++--Evaporationcloudsrainamountofwaterevaporation…ExogenousItemsItemsthataffectotheritemsinthesystembutarenotthemselvesaffectedbyanythinginthesystemArrowsaredrawnfromtheseitemsbuttherearenoarrowsdrawntotheseitemsSunlightreachingeachplantDensityofplantsSunlight++--DelaysSystemsoftenrespondsluggishlyFromtheexamplebelow,oncethetreesareplanted,theharvestratecanbe‘0’untilthetreesgrowenoughtoharvest#ofgrowingtreesHarvestratePlantingrate++--delayNextClassCreateabasiccausalgraphAugmentthecausalgraphwithmoreinformationConverttheaugmentedcausalgraphtoaSystemDynamicsflowgraphTranslateaSystemDynamicsflowgraphintoDYNAMOprogramsorequationsReferencesSimulationModelDesignandExecution,Fishwick,Prentice-Hall,1995(Textbook)IntroductiontoComputerSimulation:Asystemdynamicsmodelingapproach,NancyRobertsetal,Addison-wesley,1983BusinessDynamics:Systemsthinkingandmodelingforacomplexworld,JohnD.Sterman,McGraw-Hill,2000