Biostatistics Main 生物统计学21-30.pptx

BiostatisticsMain21 30 Q21 Inalargepopulationwithlittlemigration theincidenceofdiabetesmellitusrtypellisequalto3casesper1 000peryear andhasbeenstableforthelast30years Theprevalenceofthisdiseaseincreasedprogressivelyoverthesameperiod Whichofthefollowingcouldbethemostlikelyexplanationofthistrendovertime A HighmortalityindiabeticsB SelectivesurvivalC ImprovedqualityofcareD DecreasedhospitalizationrateE Increaseddiagnosticaccuracy A21 Correctanswer CIncidenceandprevalencearetwoImportantconceptsinepidemiology Itisthusveryimportanttoknowthedifferencebetweenthesetwo Incidenceisthemeasureoftheappearanceofnewcases Prevalenceisthemeasureofthosewiththediseaseinthepopulationataparticularpointintime Therelationshipbetweenthesetwocategoriescanbedemonstratedbythefollowingapproximationinastablepopulation littlemigration Prevalence Incidence x Time Theabovevignettedescribedadiseasewitharisingprevalencebutstableincidence Morepeoplearebeingdocumentedtohavethedisease whilethenumberofpeoplebeingdiagnosedremainsthesame Suchtrendcanbeattributedtofactorswhichprolongthedurationofthedisease e g fimprovedqualityofcare ChoiceA Ahighmortalityrateindiabeticswouldresultinadecreasedprevalence ChoicesBandD Selectivesurvivalanddecreasedhospitalizationratearenothelpfulinexplainingtheincreasedprevalencedescribedinthisscenario ChoiceE Increaseddiagnosticaccuracyaffectsboththeprevalenceandincidenceofadisease EducationalObjective AnincreasingprevalenceandstableIncidencecanbeattributedtofactorswhichprolongthedurationofadisease e g improvedqualityofcare thisscenarioistypicalfortheUSMLE Q22 Bleedingriskisthemajorconcernwhenanticoagulatingpatientswithnon valvularatrialfibrillation TheadvertisementpromotesKalaxinasasafemedication InthestudycomparingKalaxinandwarfarin whichofthefollowingspecificbleedingrisksweremostsimilarbetweenthe2groups A GastrointestinalbleedingB IntracranialbleedingC Life threateningbleedingD MajorbleedingE Totalbleeding A22 Correctanswer DThedrugadvertisementiscomparingtheeffectofKalaxinversuswarfarinforpreventingstrokeinnon valvularatrialfibrillation Itcomparesthehazardratesforadverseeventsforbothdrugsintheformofhazardratios Ahazardrateisthechanceofaneventoccurringinoneofthestudygroupsduringasetperiod Ahazardratioisthechanceofaneventoccurringinthetreatmentgroupcomparedtothechanceofthateventoccurringinthecontrolgroupduringasetperiod Ahazardratio1signifiesthataneventismorelikelytooccurinthetreatmentgroup Aratiocloseto1implieslittledifferencebetweenthe2groups Inthisstudy thehazardratioformajorbleedingwas0 9G whichistheclosestto1comparedtotheotheransweroptions Inaddition the95 confidenceinterval 0 84 1 10 containsthenullvalueof1 indicatingthatthereisnosignificantdifferenceintheriskofmajorbleedingbetweenthe2groups Thisinformationispresentedinthetextbelowthebargraph Aswithanycommercialentity drugmanufacturersmayhaveincentivetomaximizethevisibilityofadrug sbenefitsoveritsrisksorsideeffects Asaresult itisimportanttocriticallyreadallpresentedinformationwhenreviewingadrugadvertisement ChoiceA Thehazardratioforgastrointestinalbleedingis1 75 indicatingthatKalaxinhasahigherchanceofcausinggastrointestinalbleedingthanwarfarin Thehazardratioformajorgastrointestinalbleedingis1 38 ChoiceB Thehazardratioforintracranialbleedingis0 39 indicatingthatKalaxinhasalowerchanceofcausingintracranialbleedingthanwarfarin ChoiceC Thehazardratioforlife threateningbleedingis0 75rindicatingthatKalaxinhasalowerchanceofcausinglife threateningbleedingthanwarfarin ChoiceE Thehazardratiofortotalbleedingis0 91 indicatingthatKalaxinhasaslightlylowerchanceofcausingoverallbleedingthanwarfarin Educationalobjective Hazardratiosareproportionsthatindicatethechanceofaneventoccurringinthetreatmentgroupcomparedtothechanceoftheeventoccurringinthecontrolgroup Whenreviewingadrugadvertisement itisimportanttocriticallyreadallthepresentedinformation Q23 Whenassigningpatientstotreatmentandcontrolgroups properrandomizationisimportanttoavoidtheeffectofextraneousvariablesinthestudyresults Inthecurrentstudy whichofthefollowingformsofadditionalinformationwouldhelpmostindeterminingrandomizationsuccess A AnnualstrokeratesB BaselinepatientcharacteristicsC PatientcompliancechartD Patientfollow upratesE Subgroupanalysistables A23 Correctanswer BInanyrandomizedclinicalstudy thegoalofsuccessfulrandomizationistoeliminatebiasintreatmentassignments Anidealrandomizationprocessminimizesselectionbias resultsinnear equaltreatmentandcontrolgroupsizes andachievesalowprobabilityofhavingconfoundingvariables Atableofpatientbaselinecharacteristicsforbothtreatmentandcontrolgroupswouldshowifthe2groupsincludedpatientswithsimilarfeaturesandwouldhelpconfirmproperrandomization ChoiceA Annualstrokerateisanendpointofthestudythatmaybeinfluencedbypatientrandomization butitdoesnotdeterminesuccessofrandomization ChoiceC Patientcompliancechartsindicatehowwellapatienttoleratedthetreatmentandfollowedthetreatmentregimen butitwouldnotdirectlyindicatethesuccessofrandomization ChoiceD Patientfollow upratesindicatehowmanypatientsfollowedupordroppedoutofthestudy Thesedataarenotadirectmarkerofrandomizationsuccess ChoiceE Subgroupanalysistables stratifiedanalysis areusedtodetermineIfcertainpatientcharacteristics eg age comorbidities influencedtreatmenttoanysignificantextent improperrandomizationcanleadtoinaccuratesubgroupanalysisresults Educationalobjective Successfulrandomizationinaclinicaltrialallowsastudytoeliminatebiasintreatmentassignments Anidealrandomizationprocessminimizesselectionbias resultsinnear equaltreatmentandcontrolgroupsizes andachievesalowprobabilityofconfoundingvariables Q24 AlargeclinicaltrialisperformedtoevaluatethetreatmentresponsetostatinsInstatin naivepatientsadmittedforanginalchestpain Patientsarerandomlyassignedtomoderate orhigh dosestatintherapyupondischarge At3months theadversecardiovasculareventrateinthehigh dosegroupcomparedtothemoderate dosegroupwas0 98withapvalueof0 80 At1year therelativeriskratiowas0 67withapvalueof0 01 Treatmentandfollow upcompliancewashighinbothgroupsthroughoutthestudyinterval Thedifferencebetweenthe2riskestimatesisbestexplainedbywhichofthefollowing A ConfoundingbiasB LatencyperiodC ObserverbiasD SelectionbiasE Selectivesurvivalbias A24 Correctanswer BLatencyisanimportantnaturalphenomenonofdiseaseepidemiology Mostinfectiousdiseaseshaverelativelyshortlatencyperiods ie thetimeelapsedfrominitialexposuretoclinicallyapparentdisease Incontrast somediseaseprocesses eg cancer heartdisease havealonglatencyperiodbeforeclinicalmanifestationsdevelop Theconceptof3latencyperiodcanalsobeextendedtoriskfactorsandriskreducers Sometimes asignificantamountoftimemustpassbeforeexposuretoariskmodifierhasaclinicallyevidenteffectonthediseaseprocess Inaddition exposuretoariskmodifiermayneedtooccurcontinuouslyoveracertainperiodbeforethediseaseoutcomeisaffected Inthiscase atleast1yearofhigh dosestatintherapywasrequiredtoshowasignificantprotectiveadvantageovermoderate dosetherapy ChoiceA Confoundingbiasoccurswhenaperceivedexposure diseaserelationshipisactuallycausedbyanextraneousfactorthatcorrelateswithboththeexposureandthedisease Thereisnoinformationsuggestingthatconfoundingoccurredinthisstudy ChoiceC Observerbiasoccurswhenanobservermisclassifiesdataduetoindividualdifferencesininterpretationorpreconceivedexpectationsregardingthestudy Itcanbereducedbyperformingadouble blindstudywithmultipleobserverstoencodeandverifyrecordeddata ChoiceD Selectionbiascanoccurwithinappropriate ie nonrandom assignmentmethodsorthroughselectiveattritionofthestudyparticipants Itresultsinastudypopulationthatdoesnotaccuratelyrepresenttheactualpopulation leadingtoerroneousconclusionsregardingtheexposure diseaserelationship ChoiceE Selectivesurvivalbiasoccursincase controlstudieswhencasesareselectedfromtheentirediseasepopulationinsteadofjustthosethatarenewlydiagnosed Forinstance astudyoncancersurvivalthatisnotlimitedtonewlydiagnosedpatientswillcontainahigherproportionofrelativelybenignmalignanciesasthesepatientsgenerallylivelonger Educationalobjective Theconceptofalatencyperiodcanbeappliedtobothdiseasepathogenesisandexposuretoriskmodifiers Exposuretoriskfactorsandtheinitialstepsindiseasepathogenesissometimesoccuryearsbeforeclinicalmanifestationsareevident Inaddition exposuretoriskmodifiersmayneedtobecontinuousoveracertainperiodbeforeinfluencingtheoutcome Q25 Anewscreeningtestwasdevisedtodetectpancreaticcanceratearlystagesusingaserummarker CA19 9 ofthedisease Astudyofthisnewtestshowedthatitsuseprolongsthesurvivalofpatientswithpancreaticcancerbyseveralmonths Theresearchersconcludedthatuseofthetestimprovestheprognosisofpatientswithpancreaticcancer Whichofthefollowingisapotentialproblemwiththisconclusion A Observer sbiasB MeasurementbiasC Lead timebiasD ConfoundingE Ascertainmentbias A25 Correctanswer CLead timebiasshouldalwaysbeconsideredwhileevaluatinganyscreeningtest Thisbiasoccurswhenthereisanincorrectassumptionorconclusionofprolongedapparentsurvivalandbetterprognosisduetoascreeningtest Whatactuallyhappensisthatdetectionofthediseasewasmadeatanearlierpointintime butthediseasecourseitselfortheprognosisdidnotchange sothescreenedpatientsappearedtolivelongerfromthetimeofdiagnosistothetimeofdeath USMLEtip thinkoflead timebiaswhenyousee anewscreeningtest forpoorprognosisdiseaseslikelungorpancreaticcancer ChoicesA BandE Observer sbias measurementbiasandascertainmentbiasrefertomisclassificationofanoutcomeand orexposure e g labelingdiseasedsubjectsasnon diseasedandviceversa andarerelatedtothedesignofthestudy Thescenariodescribeddoesnotmentionhowthestudywasdesigned ChoiceD Althoughtheresultsofthestudycouldbepotentiallyconfounded thereisnoinformationonhowpotentialconfoundersweretreatedduringthedesignoranalysisstageofthisstudy EducationalObjective Understandtheconceptoflead timebiasinscreeningtests Thetypicalexampleoflead timebiasisprolongationofapparentsurvivalinpatientstowhomatestisapplied withoutchangingtheprognosisofthedisease Q26 Agroupofinvestigatorsisplanningastudytoevaluatetherelationshipbetweenserumfibrinogenlevelsandtheincidenceofacutecoronarysyndrome Theyassumethatserumfibrinogenlevelsarenormallydistributedinthepopulationofinterest Whichofthefollowingstatementsismostconsistentwiththisassumption A MeanisequaltomedianB MeanisgreaterthanmedianC MeanisgreaterthanmodeD MedianisgreaterthanmeanE Modeisgreaterthanmean A26 Correctanswer AThenormaldistributionisoneofthemostcommonlyoccurringstatisticaldistributions Manyvariablesinreallife eg laboratoryvalues arenormallydistributedorclosetonormal Thenormaldistributionisbell shapedandsymmetric Consequently allitsmeasuresofcentraltendencyareequal mean median mode Inreallife thevaluesarenotexactlyequalduetosmallvariations buttheylieveryclosetoeachother Skeweddistributionsdonothavethisproperty ChoicesBandC Inapositivelyskeweddistribution tailontheright themeanisgreaterthanthemedianandgreaterthanthemode ChoicesDandE Inanegativelyskeweddistribution tailontheleft themeanislessthanthemedianandlessthanthemode Educationalobjective Anormaldistributionissymmetricandbell shaped Allitsmeasuresofcentraltendencyareequal mean median mode Q27 ConsecutivereadingsofpulmonarycapillarywedgepressurewereobtainedfromapatientintheintensivecareunitusingaSwan Ganzcatheter Thereadingsare20mmHg 22mmHg 21mmHg 22mmHg and16mmHg Whichofthefollowingisthemedianofthevaluesgivenabove A 18mmHgB 20mmHgC 20 6mmHgD 21mmHgE 22mmHg A27 Correctanswer DItisimportanttoknowthedifferencebetweenthemeasuresofcentraltendency mean mode andmedian Themedianofadatasetisthenumberthatseparatestherighthalffromthelefthalfofthedataset ThedatasetMUSTbeorderedbeforethemedianisdetermined nthiscase theordereddatasetis 18 20 21r22 22 Thevalue21splitsthedatasetinhalf 2valuestotheleftand2valuestotheright therefore itisthemedian Ifthenumberofobservationsiseven findingthemedianbecomestrickier itrequiresfindingthemiddle2values addingthemtogether anddividingtheresultingsumby2 Forexample iftherewereanadditionalmeasurementof18mmHg theordereddatasetwouldconsistof6observations evennumber 18 18 20 21r22r22 Themedianvaluesplitsthedatasetinhalf itliesbetween20and21rwith3valuesontheleft 18 13 20 and3valuesontheright 21T22 22 Themedianwouldbe 20 21 2 20 5mmHg Anothermeasureofthecenterofadatasetisthemode whichistheeasiestvaluetofind Themodeisthemostfrequentvalueofadataset Inthescenariodescribed themodeis22mmHg ChoiceE Somedatasetshave 1mode ifseveralvaluesarepresentwiththesamefrequency Tofindthemean ormathematicalaverage ofadataset thesumofalltheobservationsisdividedbythenumberofobservations Inthiscase themeanisequalto 18 20 21 22 22 5 20 6mmHg ChoiceC Educationalobjective Themedianisthevaluethatislocatedinthemiddleofadataset Itdividestherighthalfofthedatafromthelefthalf Q28 An80 year oldwomanwithmultiplemedicalproblemsisadmittedtothehospitalforevaluationofsyncope Herheartrateisfoundtobe36 min Shewasadmittedtoanotherhospitalrecently thendischargedtoanacuterehabilitationfacility andthensenttoanursinghome Reviewofherhistoryshowsthatthepatientistakinghigh dosediltiazem metoprolol andatenolol Whichofthefollowingmedicationreconciliationinterventionsismostlikelytoresultinadecreaseinadversedrugeventsandhealthcareutilization A EducationalbookletsdistributedtopatientsB EducationalbookletsdistributedtophysiciansC Informationtechnology relatedinterventionsD Nurseandnurseassistant directedinterventionsE Pharmacist directedinterventions A28 Correctanswer ETransitionsofcare betweeninpatientandoutpatientfacilities0ndbetweenunitswithininpatientfacilities arepotentialsourcesofmedicalerror Inparticular continuityofcareformedicationsisespeciallyproblematic anderrorscanleadtoanincreasedriskofmedicalcomplicationsaswellascost Asaresult mostinpatientfacilitiesintheUnitedStateshaveinstitutedprotocolstoimprovetheaccuracyofthemedicationrecordduringtransitionsofcare Objectivestudiestoidentifybestpracticesformedicationreconciliationhavebeenlimited withfewinterventionsconfirmedtoimprovequalityofcare ChoicesA B C andD However interventionsthattargetpharmacypersonnelandhigh riskpatientsinparticularappeartohavethemostimpactonthequalityofpatientcare Educationalobjective Continuityofcareformedicationsatthetimeoftransitionsofcare betweeninpatientandoutpatientfacilitiesandwithininpatientfacilities isapotentialsourceofmedicalerror Interventionsthattargetpharmacypersonnelandhigh riskpatientsappeartobethemosteffectiveinimprovingthequalityofpatientcare Q29 A35 year oldCaucasianfemalepresentstotheofficeduetoaself palpatedbreastmass Aftertheappropriatework up fine needleaspiration FNA ofthemassisperformed TheresultsoftheFNAreturnasnegative Asyouareexplainingthetestresult thepatientasks WhatarethechancesthatIreallydonothavebreastcancer Whichofthefollowingvaluesbestaddressesthispatient squestion A SensitivityB SpecificityC PositivepredictivevalueD NegativepredictivevalueE Validity A29 Correctanswer DThenegativepredictivevalue NPV isdefinedastheprobabilityofbeingfreeofadiseaseifthetestresultisnegative OneveryimportantthingtorememberisthattheNPVwillvarywiththepretestprobabilityofadisease ApatientwithahighprobabilityofhavingadiseasewillhavealowNPV andapatientwithalowprobabilityofhavingadiseasewillhaveahighNPV Specificexamplesaregivenbelow 1 BreastcancerandFMAtestresultsApatientwithahighpre testprobabilityforhavingbreastcancer eg female hasfirstdegreerelativeswithbreastcancer greaterthan40yearsold hasalowNPV Apatientwithalowpre testprobabilityofhavingbreastcancer e g lessthan40yearsold asinthiscase hasahighNPV 2 HIVandELISAtestresultsApatientwhobelongstoahigh riskgroup eg multiplesexualpartners admitstonotusingcondoms IVdruguser hasahighpre testprobability consequently thispatientwillhavealowNPV Ontheotherhand apatientwhobelongstoalow riskgroup e g ronesexualpartnerwhohasnoothersexualpartners usescondomscorrectlyallthetime nohistoryofIVdruguse hasalowpre testprobability consequently thispatientwillhaveahighNFV Note Theprevalenceofadiseaseisdirectlyrelatedtothepre testprobabilityofhavingthedisease andalsoaffectstheNPV ChoicesAandB Thesensitivityandspecificityofatestarefixedvalueswhichdonotvarywiththepretestprobabilityofadisease Mostresearchersagreethattheidealdiagnostictestshouldhavehighsensitivityandspecificity inthiscase FNAalsohasahighsensitivityandspecificity however thestatisticalparameterbeingdescribedbythepatientandphysicianwasthehighnegativepredictivevalue ChoiceC Thepositivepredictivevaluefollowsthesameconcept butappliesifthelestresultispositive ChoiceE Validityrepresentstheappropriatenessofthetest i e rthetestmeasureswhatitissupposedtomeasure Itdoesnotdependonthepretestprobabilityofthedisease EducationalO