Slide 1

COHORT STUDY DESIGNNyoman Sutarsa, MD., MPHCommunity and Preventive (Public Health) DepartmentFaculty of Medicine, Udayana UniversitySutarsa_71@yahoo.comPh. +62878 6038 0028

Learning Outcomes To describe prospective and retrospective cohort studiesTo explain advantages and disadvantages of cohort studiesTo explain risk(s) interpretation in cohort studies

Reference(s): Chapter 8. Cohort Studies (Greenberg et al., 2001; pp. 113 125)

Optional:Bonita et al., 2006. Basic Epidemiology (WHO) (pp. 46-49)Webb and Bain, 2011. Essential Epidemiology (pp. 100-106)

Correction(s)Case 1. Tobacco smoking as a risk factor for depression: A 26-year population-based follow up study

Please skip this during SGD and do it as a homework instead. Please write it down on a paper and submit them tomorrow morning!

Experimental StudyObservational StudyDo the researcher assign a particular intervention?YesNoRandom AllocationComparison GroupsRCTsNon-RCTsYesNoNoYesAnalitik: Crossectional, Case-Control, CohortDescriptive: Crossectional and Longitudinal

ILLUSTRATION: Parikesit, a cardiologist, delivered a community health education related to coronary heart disease (CHD). He explained that from 100 patients who didnt regularly exercise visiting his clinic, 80 were found to have CHD. He then concluded that lack of physical activities is the major risk factor for CHD.

Please explain your comment about his conclusion!

WHAT IS COHORT?A group of people who have certain similar characteristicsExample(s):Birth CohortMarital CohortMedical Faculty Admission CohortSmoker Cohort IUD Users Cohort

COHORT STUDYComparing the exposed and the un-exposed groups, then inquiring the disease incidenceTo study the relationship between a risk factor and a disease outcome CAUSE TO EFFECTEligibleSubjects (Free of disease)ExposedUnexposedDisease (+)Disease (-)Disease (+)Disease (-)Direction of InquiryTime of StudyOnset of studyCAUSE TO EFFECT

TYPESCohort ProspectiveCohort ProspectiveCohort Retrospective (Historical Cohort)EligibleSubjectsExposedUnexposedDisease (+)Disease (-)Disease (+)Disease (-)Direction of InquiryProspective Onset of studyCAUSE TO EFFECTEligibleSubjectsExposedUnexposedDisease (+)Disease (-)Disease (+)Disease (-)Direction of InquiryRetrospectiveOnset of study (2013)CAUSE TO EFFECTSecondary Data2012

When cohort studies can be used INDICATION (s)To explore the relationship between CAUSE (s) or RISK FACTOR(s) and EFFECT (s) or DISEASESThe incidence of disease (under investigation) is quite common (not considered as a rare disease)To gather information on the incidence of diseases

STUDY PROCEDURES Define the study population (cohort population)Select the study group (exposed) and control group (un-exposed)Collect the exposure dataObserve the outcome(s)Conduct data analysis

Determining Study Group: Study Group (s) are an exposed population (with a particular risk factor) and can be evaluated RadiologistFarmers using pesticidesSmokersPeople who live in area with high level of CO or other chemical hazardsExamples:

Determining Study Group: A group who are possible to follow-upExamples:Industrial workersInsurance clientsPatients in Ante Natal Care clinicsA group of volunteer

Cohort from certain geographic area

Determining Control Group: 1. Internal Control:Control selected from the same population with the study group2. Other Cohort as a control group Control selected from a different population

Collecting the exposure data It is essential to define the exposure clearly, because some exposures are one-time episode (asphyxia at birth) and some are long term exposure (cigarette smoking, use of oral contraceptive)

Medical RecordInterviewing Cohort MemberExamining the health statusExamining Environmental factors (e.g. CO level, Lead level)

Observing the outcome(s) Information on outcome may come from various sources and it must have standard criteriaThe record of physicians and hospitalsPeriodic examination by the investigators.Combination of the two

Analysis Relative Risk (RR)Attributable Risk (AR)Attributable Risk % (AR%)

Relative Risk (RR)The ratio of incidence rate among the exposed group with the unexposed groupRisk RatioRate Ratio

RR CalculationDiseaseExposed GroupUn-exposed GroupTotalYesABA+BNoCDC+DTotalA+CB+DA+B+C+DRR = [A/(A+C)]/[B/(B+D)]

RR Interpretation Example:RR = 1,8The risk of the exposed group is 1.8 times higher to suffer from a disease compared to the unexposed group.Statistically significant?Calculate the Confidence IntervalStatistic TestHo: RR = 1Ha: RR 1 or Ha: RR > 1

Calculating CI of RR

Where:

If 1 inside the CI, then the risk between exposed and un-exposed groups is not significantly different.

Example: * RR = 1,5; 95% CI: 0,5 3,6* 1 inside the CI not significant

How to interpret CI of RR?

Analysis Relative Risk (RR)Attributable Risk (AR)Attributable Risk % (AR%)atributable Risk (AR) Example: P(D|C)= .10 (incidence among control group)P(D|E)= .40 (incidence among exposure group) AR =P(D|E) P(D|C) = .40 - .10 = .30 (incidence attributed to exposure)The incidence of exposed group minus the incidence of unexposed group

OutcomeExposedUnexposedTotalYesABA+BNoCDC+DTotalA+CB+DA+B+C+DCalculating AR and AR%:AR = [A/(A+C)] - [B/(B+D)]ARP = A/(A+C) B/(B+D)A/(A+C)x 100%

Interpretation Attributable riskincidence of the disease attributable to the exposed factor or number of incidence will reduce when the risk is not existedInterpretation Attributable risk %Proportion of all cases (disease) in the population caused by the exposure (indicator for public health impact)How to interpret AR and AR%?

Confounding in Cohort StudiesHypertensionCHDConfoundingSmokingDrinkingAgeSexControl by design?(matched or elimination)Control by analysis? (multivariate analysis)COHORT STUDIES:Advantages LimitationsIdeal to study incidence and natural history of disease, particularly those with short duration and frequentRequire long time and high costIdeal to explain the relationship between risk factors and disease/outcomes (temporal relationship)Often really complexGood to study fatal and progressive cases or diseasesMay not applicable or not efficient to rare casesCan be used to study several effects (at one point) from the risk factorDrop out or lost to follow up threat and the exposure intensity might change over time disturb data analysisGenerally are more accurate because long and continuous observationEthical consideration for a particular cases if we let the harmful exposure to be continued