presentation in epidemiology

8/8/2019 Presentation in Epidemiology

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COHORT STUDYCOHORT STUDY

PREPARED BY:PREPARED BY:MioleMiole,Mary Rose,Mary RoseOcionesOciones,, JovelJovel RoseRosePamisanPamisan, SheilaMay, SheilaMayRosario, Donna LeiRosario, Donna Lei


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CohortstudyisundertakentosupporttheCohortstudyisundertakentosupporttheexistenceofassociation betweensuspectedexistenceofassociation betweensuspected

causeanddiseasecauseanddisease A majorlimitationofcrossA majorlimitationofcross--sectionalsurveysandsectionalsurveysand

casecase--controlstudiesisdifficultyindeterminingifcontrolstudiesisdifficultyindeterminingif

exposureexposure ororriskfactorriskfactor precededtheprecededthediseasedisease oror

outcomeoutcome..

CohortStudy:CohortStudy:

Key Point:Key Point:

PresenceorabsenceofriskfactorisPresenceorabsenceofriskfactorisdetermineddetermined beforebeforeoutcomeoccurs.outcomeoccurs.


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WHAT IS COHORTWHAT IS COHORT

Ancient RomanmilitaryAncient Romanmilitaryunit, A bandofwarriors.unit, A bandofwarriors.

referstoagroupofreferstoagroupof

peoplethathaveapeoplethathaveacommonsimilarity.commonsimilarity.

Persons bandedtogether.Persons bandedtogether.

Groupofpersonswith aGroupofpersonswith a

common statisticalcommon statisticalcharacteristic. [Latin]characteristic. [Latin]

E.g.age, birth dateE.g.age, birth date,,


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C

ohortstudiesC

ohortstudies longitudinallongitudinal

ProspectivestudiesProspectivestudies

ForwardlookingstudyForwardlookingstudy IncidencestudyIncidencestudy

startswith peoplefreeofdiseasestartswith peoplefreeofdisease

assessesexposureatbaselineassessesexposureatbaseline assessesdiseasestatusatfollowassessesdiseasestatusatfollow--upup


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Itisimportanttoemphasizethatacohortstudy,

isnotanexperiment, butmerelyobservesthesubjectsofthestudywithoutinterveningexcepttoask questionsorconductphysicalexaminationsandlaboratorytestsatvariousintervals.

Obviously,theinformedconsentofallparticipantsmustbeobtained,andincohortstudiesofverylongdurationitisusuallynecessarytoobtaininformedconsentbeforeeach subsequentphase

ofthestudy.


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INDICATION OF A COHORT STUDYINDICATION OF A COHORT STUDY

WhenthereisgoodevidenceofexposureWhenthereisgoodevidenceofexposureanddisease.anddisease.

Whenexposureisrare butincidenceofWhenexposureisrare butincidenceofdiseaseis higheramongexposeddiseaseis higheramongexposed

WhenfollowWhenfollow--upiseasy,cohortisstableupiseasy,cohortisstable

Whenamplefundsareavailable

Whenamplefundsareavailable


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GeneralconsiderationwhileselectionGeneralconsiderationwhileselection

ofcohortsofcohorts Both thecohortsarefreeofthedisease.Both thecohortsarefreeofthedisease.

Both thegroupsshouldequallysusceptibleBoth thegroupsshouldequallysusceptible

todiseasetodisease Both thegroupsshould becomparableBoth thegroupsshould becomparable

DiagnosticandeligibilitycriteriafortheDiagnosticandeligibilitycriteriaforthe

diseaseshould bedefinedwellinadvance.diseaseshould bedefinedwellinadvance.


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ElementsofcohortstudyElementsofcohortstudy

SelectionofstudysubjectsSelectionofstudysubjects ObtainingdataonexposureObtainingdataonexposure

SelectionofcomparisongroupSelectionofcomparisongroup

FollowupFollowupAnalysisAnalysis


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SelectionofstudysubjectsSelectionofstudysubjects

GeneralpopulationGeneralpopulation

WholepopulationinanareaWholepopulationinanarea

A representativesampleA representativesample

Special groupofpopulationSpecial groupofpopulation

SelectgroupSelectgroup

occupationgroup / professionalgroup (Dollsstudy )occupationgroup / professionalgroup (Dollsstudy )

ExposuregroupsExposuregroups

Person havingexposuretosomephysical,chemicalorPerson havingexposuretosomephysical,chemicalorbiologicalagentbiologicalagent

e.g. Xe.g. X--ray exposuretoradiologistsray exposuretoradiologists


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ObtainingdataonexposureObtainingdataonexposure

Personalinterviews / mailed questionnairePersonalinterviews / mailed questionnaire ReviewsofrecordsReviewsofrecords

Doseofdrug,radiation,typeofsurgeryetcDoseofdrug,radiation,typeofsurgeryetc

MedicalexaminationorspecialtestMedicalexaminationorspecialtest Bloodpressure,serumcholesterolBloodpressure,serumcholesterol

EnvironmentalsurveyEnvironmentalsurvey

ByobtainingthedataofexposurewecanByobtainingthedataofexposurewecanclassifycohortsasclassifycohortsas ExposedandnonexposedandExposedandnonexposedand Bydegreeexposurewecansub classifycohortsBydegreeexposurewecansub classifycohorts


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SelectionofcomparisongroupSelectionofcomparisongroup InternalcomparisonInternalcomparison

OnlyonecohortinvolvedinstudyOnlyonecohortinvolvedinstudy

Sub classifiedandinternalcomparisondoneSub classifiedandinternalcomparisondone

ExternalcomparisonExternalcomparison MorethanonecohortinthestudyforthepurposeofMorethanonecohortinthestudyforthepurposeof

comparisoncomparison e.g.Cohortofradiologistcomparedwithe.g.Cohortofradiologistcomparedwith

ophthalmologistsophthalmologists

Comparisonwith generalpopulationratesComparisonwith generalpopulationrates

IfnocomparisongroupisavailablewecancompareIfnocomparisongroupisavailablewecancomparetheratesofstudycohortwith generalpopulation.theratesofstudycohortwith generalpopulation.


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FollowFollow--upup Toobtaindataaboutoutcometo bedeterminedToobtaindataaboutoutcometo bedetermined

(morbidityordeath)(morbidityordeath) Mailed questionnaire,telephonecalls,personalMailed questionnaire,telephonecalls,personal

interviewsinterviews PeriodicmedicalexaminationPeriodicmedicalexamination ReviewingrecordsReviewingrecords Surveillanceofdeath recordsSurveillanceofdeath records FollowupisthemostcriticalpartofthestudyFollowupisthemostcriticalpartofthestudy

SomelosstofollowupisinevitableduetodeathSomelosstofollowupisinevitableduetodeath

changeofaddress,migration,changeofchangeofaddress,migration,changeofoccupation.occupation. LosstofollowLosstofollow--upisoneofthedrawupisoneofthedraw--backofthebackofthe

cohortstudy.cohortstudy.


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ANALYSISANALYSIS

CalculationofincidenceratesamongCalculationofincidenceratesamong

exposedandnonexposedgroupsexposedandnonexposedgroups

EstimationofriskEstimationofrisk


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TypesofCohortStudyTypesofCohortStudy

ProspectivecohortstudyProspectivecohortstudy

Retrospective (historical) cohortstudyRetrospective (historical) cohortstudy

CombinationofRetrospectiveandCombinationofRetrospectiveandProspectivecohortstudy.Prospectivecohortstudy.


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CohortstudiesCohortstudies

StrengthsStrengths WecanfindoutWecanfindout

incidencerateandincidencerateandriskrisk

MorethanoneMorethanonediseaserelatedtodiseaserelatedtosingleexposuresingleexposure

canestablish causecanestablish cause--

effecteffect goodwhenexposuregoodwhenexposure

israreisrare

minimizesselectionminimizesselection

andinformation biasandinformation bias

WeaknessesWeaknesses lossestofollowlossestofollow--upup

oftenrequireslargeoftenrequireslarge

samplesample ineffectiveforrareineffectiveforrare

diseasesdiseases

longtimetocompletelongtimetocomplete

expensiveexpensive

EthicalissuesEthicalissues


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USES OF COHORT STUDYUSES OF COHORT STUDY

1.Todescribethenatural historyofthe1.Todescribethenatural historyofthe

disease.disease. 2.Toidentifythenumberofnewcases2.Toidentifythenumberofnewcases

occurring.occurring.

3.Todeterminetheetiologicfactors3.Todeterminetheetiologicfactorsassociatedwith theonsetofthedisease.associatedwith theonsetofthedisease.


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4.Investigatelateand4.Investigatelateandchronicchronic effect.effect.

5.Measurestheassociation betweenrisk5.Measurestheassociation betweenrisk

factorsanddisease.factorsanddisease.

6.Usetostudyrareexposures6.Usetostudyrareexposures


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TYPES OFCOHORT STUDYTYPES OFCOHORT STUDY

1.PROSPECTIVE COHORT1.PROSPECTIVE COHORT

--relevantexposuremayormaynothaverelevantexposuremayormaynothave

occurred bythattimethestudyisinitiatedoccurred bythattimethestudyisinitiatedbutoutcome havedefinitelynotoccurred.butoutcome havedefinitelynotoccurred.

--followedintothefuturetoassessthefollowedintothefuturetoassessthe

incidentrateofthedisease.incidentrateofthedisease. Ex: Effectofbreastfeedingamonginfants.Ex: Effectofbreastfeedingamonginfants.


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2.RETROSPECTIVE

COH

ORT2.RETROSPECTIVE

COH

ORT

theexposureandoutcomeofinteresttheexposureandoutcomeofinterest

havealreadyoccurredwhenthestudyishavealreadyoccurredwhenthestudyisinitiated.initiated.

Example: DetermineifasbestosexposureExample: Determineifasbestosexposurecauselungcancer.causelungcancer.


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Advantagesofthe Cohort Study:Advantagesofthe Cohort Study:

1.Allowcompleteinformationonthesubjects1.Allowcompleteinformationonthesubjectsexposure,including qualitycontrolofdata,exposure,including qualitycontrolofdata,andexperiencethereafter.andexperiencethereafter.

2.Provideacleartemporalsequenceof2.Provideacleartemporalsequenceofexposureanddisease.exposureanddisease.

3.Giveanopportunitytostudymultiple3.Giveanopportunitytostudymultipleoutcomesrelatedtoaspecificexposure.outcomesrelatedtoaspecificexposure.


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4.Permitcalculationofincidencerates4.Permitcalculationofincidencerates(absoluterisk) aswellasrelativerisk.(absoluterisk) aswellasrelativerisk.

5.Methodologyandresultsareeasily5.Methodologyandresultsareeasilyunderstood bynonunderstood bynon--epidemiologists.epidemiologists.

6.Enablethestudyofrelativelyrare6.Enablethestudyofrelativelyrareexposures.exposures.


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Disadvantagesofthe Cohort Study:Disadvantagesofthe Cohort Study:

Notsuitedforthestudyofrarediseases becauseaNotsuitedforthestudyofrarediseases becausea

largenumberofsubjectsisrequired.largenumberofsubjectsisrequired.

Notsuitedwhenthetime betweenexposureandNotsuitedwhenthetime betweenexposureand

diseasemanifestationisverylong,although thisdiseasemanifestationisverylong,although thiscan beovercomein historicalcohortstudies.can beovercomein historicalcohortstudies.

Exposurepatterns,forexamplethecompositionofExposurepatterns,forexamplethecompositionof

oralcontraceptives,maychangeduringthecourseoralcontraceptives,maychangeduringthecourseofthestudyandmaketheresultsirrelevant.ofthestudyandmaketheresultsirrelevant.


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Maintaining high ratesoffollowMaintaining high ratesoffollow--upcan beupcan bedifficult.difficult.

ExpensivetocarryoutbecausealargeExpensivetocarryoutbecausealargenumberofsubjectsisusuallyrequired.numberofsubjectsisusuallyrequired.

Baselinedatamay besparse becausetheBaselinedatamay besparse becausethelargenumberofsubjectsdoesnotallowforlargenumberofsubjectsdoesnotallowfor

longinterviews.longinterviews.


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MeasuresofdiseaseandassociationMeasuresofdiseaseandassociation

ThemeasureofdiseaseincohortstudiesisThemeasureofdiseaseincohortstudiesis

theincidencerate,which istheproportionoftheincidencerate,which istheproportionofsubjectswhodevelopthediseaseunderstudysubjectswhodevelopthediseaseunderstudywithinaspecifiedtimeperiod. Thenumeratorwithinaspecifiedtimeperiod. Thenumeratoroftherateisthenumberofdiseasedsubjectsoftherateisthenumberofdiseasedsubjectsandthedenominatorisusuallythenumberofandthedenominatorisusuallythenumberofpersonperson--yearsofobservation. Theincidenceyearsofobservation. Theincidenceratesforexposedandnonratesforexposedandnon--exposedsubjectsexposedsubjects

arecalculatedseparately.arecalculatedseparately. Themeasureofassociation betweenexposureThemeasureofassociation betweenexposure

anddiseaseincohortstudiesistherelativeanddiseaseincohortstudiesistherelative

risk.risk.


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TherelativeriskistheratiooftheincidenceTherelativeriskistheratiooftheincidencerateofindexsubjectstothatofcontrolrateofindexsubjectstothatofcontrol

subjects. A relativeriskof1.0signifiesthatthesubjects. A relativeriskof1.0signifiesthattheincidencerateisthesameamongexposedandincidencerateisthesameamongexposedandnonnon--exposedsubjectsandindicatesalackofexposedsubjectsandindicatesalackof

association betweenexposureanddisease. Aassociation betweenexposureanddisease. Arelativeriskoflessthan1.0providesevidencerelativeriskoflessthan1.0providesevidenceforaprotectiveeffectofexposure (theforaprotectiveeffectofexposure (theincidencerateofdiseaseamongexposedisincidencerateofdiseaseamongexposedis

lowerthannonlowerthannon--exposed) whereasarelativeexposed) whereasarelativeriskabove1.0suggeststhatexposedpeopleriskabove1.0suggeststhatexposedpeopleareathigherriskofdiseasethannonareathigherriskofdiseasethannon--exposedexposed

persons.persons.


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