Biostatistics Main 1-10.pptx

BiostatisticsMain1 10 Q1 Acohortstudywasconductedtoassesstherelationshipbetweenhighsaturatedfatconsumptionandtheoccurrenceofcolorectalcarcinomaamongwomen Agroupofwomenaged40 65wasselected Thebaselinesaturatedfatconsumptionwascalculatedusingafoodquestionnaire andthecohortwasfollowedforsevenyearsforthedevelopmentofcoloncancer Thestudyshowedthatwomenwithhighbaselinesaturatedfatconsumptionhavefourtimestheriskofcolorectalcancerina7 yearperiod comparedtowomenwithlowfatconsumption RR 4 0 95 Cl 1 5 6 5 Accordingtothestudyresults whatpercentofcolorectalcarcinomainwomenwithhighfatconsumptioncouldbeattributedtotheirdiet A 25 B 50 C 75 D 90 E 100 A1 Correctanswer CAttributableriskpercent ARP oretiologicfractionisanimportantmeasureoftheimpactofariskfactorbeingstudied ARPrepresentstheexcessriskinapopulationthatcanbeexplainedbyexposuretoaparticularriskfactor Itiscalculatedbysubtractingtheriskintheunexposedpopulation baselinerisk fromtheriskintheexposedpopulation anddividingtheresultbytheriskintheexposedpopulation ARP riskinexposed riskinunexposed riskinexposed AneasierwaytocalculatetheARPistoderiveitfromtherelativerisk RR ARP RR 1 RR Inthiscase ARP 4 0 1 4 0 0 75 75 Accordingtothestudyresults 75 ofcolorectalcarcinomainthehighconsumptiongroupwasattributabletohighsaturatedfatintake EducationalObjective ARPrepresentstheexcessriskintheexposedpopulationthatcanbeattributedtotheriskfactor Itcanbeeasilyderivedfromtherelativeriskusingthefollowingformula ARP RR 1 RR Q2 Acohortstudywasconductedtoassesstherelationshipbetweenahigh fatdietandcolorectaladenocarcinoma Thestudyconcludedthatnoassociationexistsbetweentheexposureandtheoutcomeaftercontrollingforknownriskfactors age fiberconsumption andfamilyhistoryofcancer astherelativeriskwas1 15withapvalueof0 25 Theinvestigatorsalsoreportedthat40 ofthesubjectsinthehigh fatgroupand36 ofthesubjectsinthelow fatgroupwerelosttofollow upbytheendofthestudyandwerenotincludedinthefinalanalysis Accordingtothisinformation whichofthefollowingbiasesismostlikelytobepresent A ObserverbiasB RecallbiasC ReportingbiasD SelectionbiasE Surveillancebias A2 Correctanswer DLosstofollow upisaproblemforprospectivestudiesasitcreatesthepotentialforatypeofselectionbiasKnownasattritionbias Subjectsmaydropoutofastudyforanumberofreasons butifthereasonissomehowrelatedtotheoutcome thelostsubjectswilldifferintheirriskofdevelopingtheoutcomecomparedtotheremainingsubjects Whenasubstantialnumberofsubjectsarelosttofollow up thestudymayoverestimateorunderestimatetheassociationbetweentheexposureandthedisease Inthiscase asubstantialnumberofsubjectswerelostfromboththeexposed high fat andunexposed low fat groups Peopleinthelow fatgroupmayhaveleftthestudyfordifferentreasonsthanthoseinthehigh fatgroup Forinstance maybemostsubjectsinthelow fatgroupleftduetononcompliancewithastrictlow fatdiet whilemoreindividualsinthehigh fatgroupdroppedoutduetohealth relatedproblems eg obesity diabetes cardiovasculardisease Theselectivelossofhigh risksubjectsinthehigh fatgroupwouldthencausethemeasureofassociationbetweenfatconsumptionandcolorectalcancertobeunderestimated Toreducethepotentialforattritionbiasinprospectivestudies investigatorstrytoachievehighralesoffollow up Lossestofollow upmustbereportedinstudyarticlesforreaderstoknowaboutthepotentialforselectionbias ChoiceA Observerbiasoccurswhenobserversmisclassifydataduetoindividualdifferencesininterpretationorpreconceivedexpectationsregardingthestudy Thequestiongivesnoinformationonexactlyhowoutcomeswereclassifiedandrecorded ChoiceB Recallbiasoccursinretrospectivestudieswhensubjectswithnegativeoutcomesaremorelikelythancontrolsubjectstoreportcertainexposures Thisleadstomisclassificationoftheexposurestatus likelymagnifyingtheeffectofanyexposure ChoiceC Reportingbiasoccurswhenasubjectisreluctanttoreportanexposureduetostigmaabouttheexposure eg sexualbehaviors druguse ChoiceE Surveillancebiasoccurswhentheexposedgroupundergoesincreasedmonitoringrelativetothegeneralpopulation Thistendstoincreasediseasediagnosescomparedtothegeneralpopulation Educationalobjective Losstofollow upinprospectivestudiescreatesapotentialforattritionbias asubtypeofselectionbias Whenasubstantialnumberofsubjectsarelosttofollow up thestudymayoverestimateorunderestimatetheassociationbetweentheexposureandthedisease Investigatorstrytoachievehighratesoffollow uptoreducethepotentialforattritionbias Q3 AprospectivecohortstudywasconductedtoassesstherelationshipbetweenelevatedC reactiveproteinlevel exposure andthedevelopmentofacutecoronarysyndrome disease Aftera5 yearfollow upperiod thefollowingdatawereobtained Whatisthe5 yearriskofgettingacutecoronarysyndromeinsubjectswithelevatedC reactiveproteinlevels A 0 5B 0 3C 0 25D 0 2E 0 14 A3 Correctanswer ARiskisameasureofincidenceofadisease Itshowstheprobabilityofgettingadiseaseoveracertainperiodoftime 5yearsinthisscenario Tocalculatetherisk dividethenumberofdiseasedsubjectsbytheoverallnumberofsubjectsatrisk Inthisexample togetthe5 yearriskofacutecoronarysyndromeinsubjectswithelevatedC reactiveproteinlevels exposed dividethenumberofexposeddiseasedsubjects 10 bythetotalnumberofexposedpeople 20 10 20 0 5Thecalculatedriskvalueis0 5rwhichisinterpretedasa50 probabilityofgettingacutecoronarysyndromein5years ChoiceB The5 yearriskofgettingacutecoronarysyndromeinallstudysubjectsis30 100 0 3 30 ChoiceC The5 yearriskofgettingacutecoronarysyndromeinpeoplewithoutelevatedC reactiveproteinlevelsis20 80 thatis 0 25or25 ChoiceD Theprevalenceofexposureintheentirecohort exposedandunexposedcombined is20 100 thatis 0 2 ChoiceE Theprevalenceofexposureinnon diseasedpeopleis10 70 thatis 0 14 EducationalObjective Riskistheprobabilityofgettingadiseaseoveracertainperiodoftime Tocalculatetherisk dividethenumberofdiseasedsubjectsbythetotalnumberofsubjectsatrisk i e allthepeopleatrisk Q4 Astudywasconductedtoassesstheassociationbetweenhormonereplacementtherapy HRT inpost menopausalwomenandthelevelofserumC reactiveprotein CRP Thedatafromthestudyarepresentedbelow WhichofthefollowingisthebeststatisticalmethodtoassesstheassociationbetweenHRTandelevatedCRPlevels A Two samplez testB Two samplet testC Chi squaretestD ANOVAE Meta analysis A4 Correctanswer CThechi squaretestisusedtocomparetheproportionsofacategorizedoutcome 丨nthiscase theoutcome serumCRPlevel iscategorizedaseitherlhhigh and normal 1andthenpresentedwiththeexposure irHRT or noHRTM ina2x2table Inoneofthecommonlyusedchi squaretestsrtheobservedvaluesineachofthecellsarecomparedtoexpected underthehypothesisofnoassociation values Ifthedifferencebetweentheobservedandexpectedvaluesislarge anassociationbetweentheexposureandtheoutcomeisassumedtobepresent ChoicesAandB Thetwo samplez testandtwo samplet testareusedtocomparetwomeans notproportions ChoiceD Analysisofvariance ANOVA isusedtocomparethemeansofthreeormorevariables ChoiceE Meta analysisisanepidemiologicmethodofpoolingthedatafromseveralstudiestodoananalysishavingarelativelybigstatisticalpower EducationalObjective Thechi squaretestisusedtocompareproportions A2x2tablemaybeusedtocomparetheobservedvalueswiththeexpectedvalues Q5 Aprospectivecohortstudyrevealedastrongpositiveassociationbetweensmokingandlivercirrhosis relativerisk 2 8 Theresearchersthendividedthecohortintotwogroups alcoholconsumersandnon consumers Subsequentstatisticalanalysisdidnotrevealanyassociationbetweensmokingandlivercirrhosiswitheithergroup Thescenariodescribedaboveisanexampleofwhichofthefollowing A SelectionbiasB Observer sbiasC MeasurementbiasD RecallbiasE Confounding A5 Correctanswer EConfoundingreferstothebiasthatresultswhentheexposure diseaserelationshipismixedwiththeeffectofextraneousfactors j e confounders Confoundersinfluenceboththeexposureandoutcome Inthegivenstudy crudeanalysisofthedatainitiallyrevealedanassociationbetweensmokingandlivercirrhosis Themostlikelypotentialconfounderisconcomitantalcoholconsumptioninpeoplewhosmoke Itisawell knownfactthatalcoholconsumptionisstronglyassociatedwithlivercirrhosis Furthermore alcoholconsumptioncanexplainatleastpartoftheassociationobservedbetweensmokingandlivercirrhosis Methodstodealwithconfoundingincludematchingofcasesandcontrolsbasedontheconfoundingfactor orstratificationofthestudypopulationbasedontheconfoundingfactor Inthiscase runningseparateanalysesforalcoholconsumersandnon consumers thistechniqueiscalledstratifiedanalysis canunmaskconfoundinganddisclosethetrueunconfoundedvalueoftheRR ChoiceA Selectionbiasresultsfromthemannerinwhichpeopleareselectedforthestudy orfromtheselectivelossesfromfollow up Thescenariodoesnotmentionanyoftheseproblems ChoicesBandC Observer sbiasandmeasurementbiasdistortthemeasureofassociationbymisclassifyingexposed unexposedand ordiseased non diseasedsubjects Thescenariodoesnotdescribethisclassificationprocess ChoiceD Recallbiasresultsfromtheinaccuraterecallofpastexposurebypeopleinthestudy Itappliesmostlytocase controlstudies notcohortstudies EducationalOjective Knowtheconceptofconfounding Distinguishbetweencrudeandadjustedmeasuresofassociation Confoundingreferstothebiasthatcanresultwhentheexposure diseaserelationshipismixedwiththeeffectofextraneousfactors i e confounders Q6 Acase controlstudywasconductedtoevaluatetheassociationbetweenalcoholconsumptionandcanceroftheoralcavity Thecrudeanalysisshowedastrongassociationbetweentheexposureandoutcome oddsratio 4 5 95 confidenceinterval3 4 5 6 Smokingwasconsideredasapotentialconfounderoftheassociation Whichofthefollowingpropertiesofsmokingisessentialinordertobeconsideredasaconfounder A ItshouldnotberelatedtocanceroftheoralcavityB ItshouldbehighlyprevalentinthepopulationofinterestC ItshouldberelatedtoalcoholconsumptionD ItshouldbeobservedonlyinalcoholconsumersE Itshouldbeevenlydistributedamongalcoholconsumersandnon consumers A6 Correctanswer CConfoundingisdefinedasmixingupoftheeffectofexposurewiththeeffectofanextraneousfactor confounder Inordertobeaconfounder theextraneousfactormusthavesomepropertieslinkingitwiththeexposureandoutcomeofinterest Inthisexample smokingisthepossibleconfounder Smokingisrelatedtotheexposure i e peoplewhoconsumealcoholaremorelikelytosmoke aswellastheoutcomeofinterest i e smoking byitself isassociatedwithcanceroftheoralcavity Ifsmokingistheconfounder itexplainsatleastpartoftheassociationobservedbetweenalcoholconsumptionandcanceroftheoralcavity ChoiceA Ifsmokingisnotrelatedtocanceroforalcavity itcannotbeaconfounderinthiscase ChoiceB Theprevalenceofafactorinthepopulationisnotrelatedtoitseffectasaconfounder ChoiceDandE Aconfounderiscommonlyobservedbothinexposedandunexposedsubjectsanditisnotevenlydistributedbetweenthegroups EducationalObjective Aconfounderisanextraneousfactorwhichhaspropertieslinkingitwiththeexposureandoutcomeofinterest Q7 Acase controlstudywasconductedtoassesstherelationshipbetweenalcoholconsumptionandbreastcancer First theinvestigatorsInterviewedpatientswithbreastcancer Theythenselectedtheneighborsofthepatientswiththesameageandrace andusedthemascontrols Suchstudydesignhelpstocontrolwhichofthefollowingproblems A SelectionbiasB RecallbiasC Observer sbiasD AscertainmentbiasE Confounding A7 Correctanswer EMatchingisanefficientmethodtocontrolconfounding Itisfrequentlyusedincase controlstudies Theinitialstepinvolvestheselectionofmatchingvariables whichshouldalwaysbethepotentialconfounders e g rage race Casesandcontrolsarethenselectedbasedonthematchingvariables suchthatbothgroupshaveasimilardistributioninaccordancewiththevariables Inthisscenario neighborhood controls werematchedto cases byageandrace Selectingneighbors3Scontrolsh3sanotheradvantageofmatchingthecasestocontrolsbyvariablesthataredifficulttomeasure eg socioeconomicstatus ChoiceA Selectionbiasisnotcontrolledbymatching becausethecontrolsselectedmaynotreflecttheexposureexperience inthiscase alcoholconsumption ofthegeneralpopulation ChoicesB CandD Recallbias observer sbias andascertainmentbiasresultfrommislabelingexposed unexposedorcases controls Thesearenotaffectedbymatching EducationalObjective Matchingisfrequentlyusedincase controlstudiesbecauseiiisanefficientmethodtocontrolconfounding Remember matchingvariablesshouldalwaysbethepotentialconfoundersofthestudy e g Page race Casesandcontrolsarethenselectedbasedonthematchingvariables suchthatbothgroupshaveasimilardistributioninaccordancewiththevariables Q8 Aclinicalstudyisconductedtoassesstheroleofavasopressinantagonistinoverallsurvivalofpatientswithadvancedheartfailureandhyponatremia Afterprovidinginformedconsent eligiblehospitalizedpatientsareadministeredeitherthenewdrugoramatchingplaceboalongwiththeappropriatestandardcare Assignmenttothe2treatmentarmsisdonerandomlyusingcomputer generatednumbers Thisassignmentstrategyismosthelpfulforcontrollingwhichofthefollowing A ConfoundingB EffectmodificationC Post hocanalysisD RecallbiasE Selectivesurvival A8 Correctanswer ATheeffectsofconfoundingbiascanbereducedthroughgoodstudydesignandproperanalysisoftheresults Methodsusedtocontrolforconfoundingduringdataanalysisincludestratifiedanalysisandstatisticalmodeling eg multivariateanalyses Inaddition methodstocontrolforconfoundersduringstudydesignincludethefollowing 1 Matching Frequentlyusedincase controlstudies thismethodinvolvesmatchingknownorsuspectedconfoundingvariablesbetweenthecasesandcontrols Forexample ifsmokingstatusisaknownconfounderthencontrolswillbeselectedonacase by casebasissothattherearesimilarnumbersofsmokersandnonsmokersinboththecaseandcontrolgroups 2 Restriction ThismethocHnvolveslimitingstudyparticipationtoindividualswithspecificcharacteristics Forexample ifsexisthoughttobeaconfounder thenarestrictedstudymayenrollonlymen However thismethodislimitedasthestudyresultsmaynotbeapplicabletotheexcludedpopulation ie women 3 Randomization Commonlyemployedinclinicaltrials thistechniquehelpstobalancethedistributionofconfoundingvariablesbetweentreatmentandplacebogroupssothattheunconfoundedeffectoftheexposureofinterestcanbeisolated Animportantadvantageofrandomization comparedtoothermethods isthepossibilityofcontrollingknownriskfactors egrageandseverityofheartfailure aswellastheunknownanddifficult to measureconfounders eg levelofstressandsocioeconomicstatus Randomizationalsoeliminatespartialityintreatmentassignments minimizingselectionbias ChoiceB Effectmodificationresultswhenanexternalvariablepositivelyornegativelyimpactstheeffectofariskfactoronthediseaseofinterest Forinstance theriskofvenousthrombosisisincreasedwithestrogentherapy andthiseffectisaugmentedbysmoking ChoiceC Post hocanalysisreferstoperformingunplannedstatisticaltestsonpatternsthatwereidentifiedafterthefactindatafromacompletedstudy Thiscanleadtoincorrectconclusions particularlyiftheappropriatestatisticalmeasureshavenotbeentakentoaccountfortheseadditionaltests Post hocanalysiscanbeproblematicwithnon predefinedsubgroupanalysis Randomizationdoesnotdirectlyimpactpost hocanalysis whichcanbeconductedevenondatafromrandomizedsamples ChoiceD Recallbiasresultsfrominaccuraterecallofpastexposureandappliesprimarilytoretrospectivestudies Peoplewhohavesufferedanadverseeventaremorelikelytorecallriskfactorsthanpeoplewithoutsuchexperiences ChoiceE Selectivesurvivalbiasoccursincase controlstudieswhencasesareselectedfromtheentirediseasepopulationinsteadofjustthosethatarenewlydiagnosed Forinstance astudyoncancersurvivalthatisnotlimitedtonewlydiagnosedpatientswillcontainahigherproportionofrelativelybenignmalignanciesasthesepatientsgenerallylivelonger Educationalobjective Randomizationisusedtocontrolforconfoundersduringthedesignstageofastudy helpstocontrolforknown unknown anddifficult to measureconfounders Q9 AstudywasconductedtoassesstherelationshipbetweenserumHDL2level asubtractionofHDL andcarotidintima mediathicknessasamarkerofatherosclerosis Interpretationoftheresultsrevealedalinearrelationshipbetweenthese2variables withacorrelationcoefficientr 0 25 p 0 005 Whichofthefollowingstatementsabouttheresultsofthestudyisthemostcorrect A AsHDL2levelincreases thereisanassociateddecreaseincarotidintima mediathicknessB DecreasedHD1 2levelisthecauseofcarotidintima mediathickeningC TheassociationisnotstatisticallysignificantD ThereisastrongcorrelationbetweenHDL2levelandcarotidintima mediathicknessE ThereisapositivecorrelationbetweenHDL2levelandcarotidintima mediathickness A9 Correctanswer AThecorrelationcoefficient r assessesalinearrelationshipbetween2variables Thenullvalueforthecorrelationcoefficientis0 noassociation andthesignofthecorrelationcoefficientindicatesapositiveornegativeassociation Thecloserristoitsmargins 1or1 fthestrongertheassociation Inthisstudy astherewasanegativecorrelationbetweenHDL2andcarotidintima mediathickness givenr 0 250 5orr0 wouldhavemeantthatincreasesinHDL2wereassociatedwithincreasesincarotid intimathickness Educationalobjective Thecorrelationcoefficient r showsthestrengthanddirection positive negative oflinearassociationbetween2variables ltdoesno