How Complex Systems Fail

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1、HowSystemsFailHowComplexSystemsFail(BeingaShortTreatiseontheNatureofFailure;HowFailureisEvaluated;HowFailureisAttributedtoProximateCause;andtheResultingNewUnderstandingofPatientSafety)RichardI.Cook,MDCognitivetechnologiesLaboratoryUniversityofChicago1)Complexsystem

2、sareintrinsicallyhazardoussystems.Alloftheinterestingsystems(e.g.transportation,healthcare,powergeneration)areinherentlyandunavoidablyhazardousbytheownnature.Thefrequencyofhazardexposurecansometimesbechangedbuttheprocessesinvolvedinthesystemarethemselvesintrinsical

3、lyandirreduciblyhazardous.Itisthepresenceofthesehazardsthatdrivesthecreationofdefensesagainsthazardthatcharacterizethesesystems.2)Complexsystemsareheavilyandsuccessfullydefendedagainstfailure.Thehighconsequencesoffailureleadovertimetotheconstructionofmultiplelayers

4、ofdefenseagainstfailure.Thesedefensesincludeobvioustechnicalcomponents(e.g.backupsystems,‘safety’featuresofequipment)andhumancomponents(e.g.training,knowledge)butalsoavarietyoforganizational,institutional,andregulatorydefenses(e.g.policiesandprocedures,certificatio

5、n,workrules,teamtraining).Theeffectofthesemeasuresistoprovideaseriesofshieldsthatnormallydivertoperationsawayfromaccidents.3)Catastropherequiresmultiplefailures–singlepointfailuresarenotenough..Thearrayofdefensesworks.Systemoperationsaregenerallysuccessful.Overtcat

6、astrophicfailureoccurswhensmall,apparentlyinnocuousfailuresjointocreateopportunityforasystemicaccident.Eachofthesesmallfailuresisnecessarytocausecatastrophebutonlythecombinationissufficienttopermitfailure.Putanotherway,therearemanymorefailureopportunitiesthanoverts

7、ystemaccidents.Mostinitialfailuretrajectoriesareblockedbydesignedsystemsafetycomponents.Trajectoriesthatreachtheoperationallevelaremostlyblocked,usuallybypractitioners.4)Complexsystemscontainchangingmixturesoffailureslatentwithinthem.Thecomplexityofthesesystemsmake

8、sitimpossibleforthemtorunwithoutmultipleflawsbeingpresent.Becausetheseareindividuallyinsufficienttocausefailuretheyareregardedasminorfactorsdurin