2 IntroductionAll studies involve some descriptive or analytic type of comparison of exposure and disease status.Analytical study design options include: observational or interventional (which one is based on the role of the investigator).There are three basic types of observational analytical study designs:Cohort studiesCase-control studiesCross-sectional studies
3 Introduction First, using observational analytical studies : Second: Specific epidemiological study designs can be used to reveal etiologic (causal) relationshipsFirst, using observational analytical studies :Determine whether there is an association between a factor or a characteristic and the development of diseaseSecond:From these associations, derive appropriate inferences regarding a possible causal relationship
4 Analytical Studies Control and experimental groups Randomized groups data collected without biasDependent and independent factors
5 Cohort Studies Group by common characteristics Start with a group of subjects who lack a positive history of the outcome of interest yet are at risk for it (cohort).Think of going from cause to effect.The exposure of interest is determined for each member of the cohort and the group is followed to document incidence in the exposed and non-exposed members.
6 When is a cohort study warranted? When good evidence suggests an association of a disease with a certain exposure or exposures.
7 Cohort EffectChanges and variation in the disease or health status of a study population as the study group moves through time.“Generation effect”
8 Types of Cohort Studies Prospective (concurrent)Retrospective (historical)Restricted (restricted exposures)
9 Types of Cohort Studies Prospective – cohort characterized by determination of exposure levels (exposed vs. not exposed) at baseline (present) and followed for occurrence of disease in futureGroups move through time as they ageRetrospective - makes use of historical data to determine exposure level at some baseline in the past and then determine subsequent disease status in the present.Restricted - limited exposure, narrow behavior (military, long shore men)
10 Prospective Studies Also called Looking into the future Example: longitudinalconcurrentincidence studiesLooking into the futureExample:Framingham Study of coronary heart disease (CHD)
11 Design of a Cohort Experiment The essential characteristic in the design of cohort studies is the comparison of outcome in an exposed group and a nonexposed group (or a group with a certain characteristic and a group w/o that characteristic).A study population can be chosen by selecting groups for inclusion in the study on the basis of whether or not they were exposed
12 Selection of Cohort Groups There are two basic ways to generate cohort groups.Select a cohort (defined population) BEFORE any of its members become exposed or before the exposures are identified.Select a cohort on the basis of some factor (e.g., where they live) and take histories (e.g., blood tests) on the entire population to separate into exposed and non-exposed groups.Regardless of which selection approach is used, we are comparing exposed and non-exposed persons.
14 Design of a Prospective Cohort Experiment Major problem with a prospective cohort design is that the cohort must be followed up for a long period of time.
15 Framingham StudyDesigned to study the effect of multiple factors on coronary heart disease (CHD):agehypertensionelevated blood cholesteroltobacco smokingincreased physical activityincrease in body weightdiabetes mellitus
16 Framingham Study Design Framingham, Massachusetts population was 28,000Study design called for a random sample of 6,500Enrollment questionnaire form targeted age range yearsNo clinical evidence of atherosclerotic cardiovascular diseaseCohort re-examined every two yearsProblems: white, middle class
17 Hypothetical Cohort Study Approach Examine people over their life times for contraction of diseases. In meantime, ask questions concerning diet, lifestyle,habits, work, etc.Take blood and do lab tests (as many as possible).Do same tests every year for a period of years.Interventional aspect: If positive findings occur, they refer you to a specialist. By screening, they may have prolonged a life.
20 Sampling Valid, reliable surveys Critical number of subjects Randomize Garbage in,garbage outValid, reliable surveysCritical number of subjectsthe more, the betterRandomizerandom selectionrandom assignmentRule out biasFor example, degree of accuracy with which subjects have been classified with respect to their exposure.For example, individuals who are sick may be more likely to give the kind of responses that they believe the investigator wants to hear
21 Data Gathering Person - to - person Drop off questionnaire Mailed to peopleTelephone interviewNewsletter or magazine
22 Potential Biases in Cohort Studies Information biasBias in estimation of the outcomeBias from non-responseBias from losses to follow-upAnalytic biasYour assignment: Describe and differentiate between these types of biases.
23 Advantages of Prospective Cohort Studies Captive groupsLarge sample sizesCertain diseases or risk factors targetedCan be used to prove cause-effectAssess magnitude of riskBaseline of ratesNumber and proportion of cases that can be prevented
24 Advantages of Prospective Studies (cont’d) Completeness and accuracyOpportunity to avoid condition being studiedQuality of data is highConsiders seasonal and other variations over a long periodTracks effects of aging process
25 Disadvantages of Prospective Cohort Studies Large study populations requirednot easy to find subjectsExpensiveUnpredictable variablesResults not extrapolated to general populationStudy results are limitedTime consuming/results are delayedRequires rigid design and conditions
26 Disadvantages of Prospective Studies (cont’d) Subjects lost over time (dropouts)Costs are highLogistically demandingMaintaining quality, validity, accuracy and reliability can be a problem
27 Survivorship StudiesSurvivorship is the number of persons out of a study population who would survive until a certain time interval has been reachedShows the chance that an event (such as death from cancer) will occur in successive intervals of time once a diagnosis has been madeAnalysis yields a cumulative probability of surviving the projected time periodFor infectious diseases, we use case fatality rate to assess survivalFor chronic diseases, we use cohort life tables
28 (Cohort) Life TablesCharts which summarize the patterns of survival and death in study groups of certain types of disease (chronic)Insurance companies study these charts very closely.
29 Survival curves and risk of death for males vs Survival curves and risk of death for males vs. females based on life tables in California for 1980.Dip at beginning of life is due to infant mortality rate.As one reaches the later years of life, the survival curve goes down and the risk of death goes up.