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Cohort, case-control & cross- sectional studies Source: Alain Moren, EPIET Introductory courses Kostas Danis EPIET Introductory course Menorca, Spain 16/9-12/10/2012

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Epidemiological studies Two types Observation Experiment

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Exposed Not exposed Disease occurrence Unethical to perform experiments on people if exposure is harmful Exposure assigned

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Randomised Controlled Trial Blinded Doses Time period Risk - effect No bias If exposure not harmful Treatment Preventive measure (vaccination) If RCT not possible Left with observation of experiments designed by Nature Cohort studies Cross-sectional studies Case control studies

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Cohort studies marching towards outcomes

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What is a cohort? One of 10 divisions of a Roman legion Group of individuals - sharing same experience - followed up for specified period of time Examples - EPIET cohort birth cohort - cohort of guests at barbecue - occupational cohort of chemical plant workers - influenza vaccinated in

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follow-up period

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Calculate measure of frequency Cumulative incidence - incidence proportion - attack rate (outbreak) Incidence rate end of follow-up

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Cohort studies Purpose - Study if an exposure is associated with outcome(s) - Estimate risk of outcome in exposed and unexposed cohorts - Compare risk of outcome in two cohorts Cohort membership - Being at risk of outcome(s) studied - Being alive and - Being free of outcome at start of follow-up

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unexposed exposed Cohort studies

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unexposed exposed Incidence among exposed Incidence among unexposed Cohort studies

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ate ham did not eat ham illnot ill Total a b a+b c d c+d Presentation of cohort data: 2x2 table Risk in exposed= a/a+b Risk in unexposed= c/c+d

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Incidence rate Number of NEW cases of disease Total person - time of observation

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Incidence rate Number of NEW cases of disease Total person - time of observation Rate Denominator: - is a measure of time - the sum of each individuals time at risk and free from disease

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ABCDEABCDE Time at risk x x Total years at risk time followed x disease onset Person-time

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ABCDEABCDE Time at risk x x Total years at risk time followed x disease onset Incidence rate (IR) (Incidence density) IR = 2 / 35.5 person years = cases / person year = 5.6 cases / 100 person years = 56 cases / 1000 person years

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Person-yearsCases Smoke 102, Do not smoke 42,800 3 Presentation of cohort data: Person-years at risk Tobacco smoking and lung cancer, England & Wales, 1951 Source: Doll & Hill

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Presentation of data: Various exposure levels

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time ExposureStudy starts Disease occurrence Prospective cohort study time ExposureStudy starts Disease occurrence

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Retrospective cohort study Exposure time Disease occurrence Study starts

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Recipe: Cohort study Identify group of - exposed subjects - unexposed subjects Measure incidence of disease Compare incidence between exposed and unexposed group

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unexposed exposed Incidence among exposed Incidence among unexposed Cohort studies

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Absolute measures - Risk difference (RD) I e - I ue Relative measures - Relative risk (RR) Rate ratio Risk ratio Effect measures in cohort studies I e I ue I e = incidence in exposed I ue = incidence in unexposed

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Exposed Not exposed Cases Non cases Risk % Cohort study % % Risk ratio 50% / 10% = 5 Total 100

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ate ham did not eat ham illnot ill Incidence % % Risk difference 50% - 40% = 10% Relative risk 50% / 40% = 1.25

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Interpretation of Risk Ratios RR>1 RR=1 RR<1 Protective factor Risk factor No association

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Does HIV infection increase risk of developing TB among drug users?

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Vaccine efficacy (VE) VE = 1 - RR = = 72%

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Population Cases Incidence a1a1 High N1N1 I 1 c Unexposed N ne I ue at risk Exposure level a2a2 Medium N2N2 I 2 a3a3 Low N3N3 I 3 Various exposure levels

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Population Cases Incidence RR a1a1 High N1N1 I 1 c Unexposed N ne I ue at risk Exposure level a2a2 Medium N2N2 I 2 a3a3 Low N3N3 I 3 RR 1 RR 2 RR 3 Reference Various exposure levels

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Cohort study: Tobacco smoking and lung cancer, England & Wales, 1951 Source: Doll & Hill

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Disadvantages of cohort studies Large sample size Latency period Cost Time-consuming Loss to follow-up Exposure can change Multiple exposure = difficult Ethical considerations

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Strengths of cohort studies Can directly measure - incidence in exposed and unexposed groups - true relative risk Well suited for rare exposure Temporal relationship exposure-disease is clear Less subject to selection biases - outcome not known (prospective)

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Can examine multiple effects for a single exposure PopulationOutcome 1Outcome 2Outcome 3 exposedN e I e1 I e2 I e3 unexposedN ne I ue1 I ue2 I ue3 RR 1 RR 2 RR 3 Cohort studies

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A cohort study allows to calculate indicators which have a clear, precise meaning. The results are immediately understandable.

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Cross-sectional (prevalence) studies

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Cross-sectional studies Observation of a cross-section of a population at a single point in time Recruitment of study participants Population Population sample Observation for the presence of: One or more outcomes One or more exposures

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Sampling Sample Target Population Sampling Population

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Uses of cross-sectional surveys in public health Estimate prevalence of disease or their risk factors Estimate burden Measure health status in a defined population Plan health care services delivery Set priorities for disease control Generate hypotheses Examine evolving trends - Before / after surveys - Iterative cross-sectional surveys

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Potential objectives of a cross sectional study Descriptive Estimate prevalence Analytic Compare the prevalence of a disease in various subgroups, exposed and unexposed Compare the prevalence of an exposure in various subgroups, affected and unaffected

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IllNon illTotal Exposedab a+b Non exposedcd c+d Presentation of the data of an analytical cross sectional study in a 2 x 2 table Simultaneous measurement of outcomes and exposures

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Exposed Not exposed Cases Non cases Prevalence % Cross-sectional study % % Prevalence ratio (PR) 50% / 10% = 5 Total 1,000

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Measuring association in analytical cross-sectional surveys Prevalence among exposed / prevalence among unexposed Prevalence ratio Formula equivalent to risk ratio Concept different - No incidence - Only prevalence depends on both occurrence of new cases & duration of disease

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Prevalence of West Nile virus (WNV) infection by place of residence, Central Macedonia,Greece, 2010 InfectedTotal Prevalence ratio Rural %5.9 Urban %Ref

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Prevalence of HIV infection by socioeconomic status, African country X, 1999 InfectedTotal Prevalence ratio High class %2.6 Low class %Ref

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Prevalence of hepatitis C (HCV) infection by quantity of therapeutic injections, Hazabad, Pakistan, 1993 No.of injection s InfectedTotal Prevalence ratio > % % %Ref

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Advantages of cross-sectional surveys Fairly quick Easy to perform Less expensive Adapted to chronic diseases

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Limitations of cross-sectional surveys Limited capacity to document causality (exposure and outcome measured at the same time -difficult to establish time sequence of events) Not useful to study disease etiology Not suitable for the study of rare / short diseases Not adapted to severe / acute diseases Not adapted to incidence measurement.

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Limitations of causal inference in analytical cross sectional studies Prevalent cases Exposure and outcome examined at the same time

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Principle of case control studies

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Our objective is to compare the incidence rate in the exposed population to the rate that would have been observed in the same population, at the same time if it had not been exposed

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Exposed Unexposed Source population

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Cases Exposed Unexposed Source population

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Cases Exposed Unexposed Source population Sample Controls

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Cases Exposed Unexposed Source population Controls: Sample of the denominator Representative with regard to exposure Controls Sample

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Intuitively if the frequency of exposure is higher among cases than controls then the incidence rate will probably be higher among exposed than non-exposed

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Case control study Disease Controls Exposure ???? Retrospective nature

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CasesControls Exposeda b Not exposedc d Totala + c b + d % exposed Distribution of cases and controls according to exposure in a case control study

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CasesControls Exposeda b Not exposedc d Totala + c b + d % exposeda/(a+c) b/(b+d) Distribution of cases and controls according to exposure in a case control study

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Oral Myocardial contraceptivesInfarction Controls Yes No Total % exposed69.3% 32% Distribution of myocardial infarction by oral contraceptive use in cases and controls

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PhysicalMyocardial activityInfarction Controls >= 2500 Kcal < 2500 Kcal Total % exposed51.9%62.8% Distribution of myocardial infarction by amount of physical activity in cases and controls

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WaterCases Controls Consumption YES 150 ? NO 50 ? Total Volvo factory, Sweden, 3000 employees, Cohort study 200 cases of gastroenteritis

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Two types of case control studies Exploratory New disease New risk factors Several exposures "Fishing expedition" Analytical Define a single hypothesis Dose response

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Cohort studies Rate/risk Rate/risk difference Rate Ratio/Risk ratio (strength of association) No calculation of rates/risks Proportion of exposure Case control studies Any way of estimating measures of association?

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Odds Probability that an event will happen Probability that an event will not happen Probability that cases/controls will be exposed Probability that cases/controls will not be exposed

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Exposed Not exposed Cases Controls Odds ratio Case control study a b c d Total a+c OR= (a/c) / (b/d) = ad / bc a/c b/d Odds of exposure b+d % exposed a/(a+c) % unexposed c/(a+c) b/(b+d) d/(b+d)

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Exposed Not exposed Cases Controls Odds ratio Case control study a b c d Total 100 OR= (a/c) / (b/d) = ad / bc = (50x80) / (20x50) = 4 50/5020/80 Odds of exposure

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Case control study design CasesControls E E a b c d ab a x d = cdb x c Odds ratio

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Frequency of chicken consumption in campylobacter cases and controls, Republic of Ireland and Northern Ireland, 2003 CasesControls Odds ratio Ate chicken Did not eat chicken 1544Ref

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Frequency of contact with a dog in campylobacter cases and controls, Republic of Ireland and Northern Ireland, 2003 CasesControls Odds ratio Contact with dog Yes No158201Ref

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Oral Myocardial contraceptivesInfarctionControlsOR Yes No Ref. Total Odds 693/307= 320/680= of exposure Distribution of myocardial infarction by recent oral contraceptive use in cases and controls

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PhysicalMyocardial activityInfarctionControlsOR >= 2500 Kcal < 2500 Kcal176136Ref. Total odds of 190/176= 230/136= exposure Distribution of myocardial infarction by amount of physical activity in cases and controls

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Distribution of cases of endometrial cancer by oestrogen use in cases and controls Oestrogen useCasesControlsOdds ratio High a1b1a1d/b1c Low a2b2a2d/b2c None cdReference

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Relation of hepatocellular adenoma to duration of oral contraceptive use in 79 cases and 220 controls Months of OC useCasesControlsOdds ratio Ref >= Total79220 Source: Rooks et al. 1979

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Advantages of case control studies Rare diseases Several exposures Long latency Rapidity Low cost Small sample size Available data No ethical problem

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Limitations of case-control studies Cannot compute directly risk Not suitable for rare exposure Temporal relationship exposure-disease difficult to establish Biases control selection - recall biases when collecting data Loss of precision due to sampling

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The cohort study is the gold-standard of analytical epidemiology CASE-CONTROL STUDIES HAVE THEIR PLACE IN EPIDEMIOLOGY, but if cohort study possible, do not settle for second best

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Thank you!

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Back-up slides

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I 1 = a / P 1 I 0 = c /P 0 E E a c P1P1 P0P0 Population denominator Cases E E a c P 1 /10 P 0 /10 Population sample Cases a/P 1 I 1 / I 0 = c/P 0 } a I 1 = P 1 / 10 c I 0 = P 0 /10 } a/P 1 I 1 / I 0 = c/P 0

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I 1 = a / P 1 I 0 = c /P 0 CasesControls E E a b cd E E a c P1P1 P0P0 Source population Pop.Cases P 1 b --- = ---- P 0 d = sample a/P 1 I 1 / I 0 = c/P 0 }

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I 1 = a / P 1 I 0 = c /P 0 Cases = sample E E a b cd Since d/b = P 0 / P 1 E E a c P1P1 P0P0 Source population Pop.Cases a/P 1 a. P 0 a. d I 1 / I 0 = = = = c/P 0 c. P 1 c. b } Controls P 1 b --- = ---- P 0 d a / c b / d

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