Cohort StudiesIntroduction • All 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 studies Case-control studies Cross-sectional studies Introduction • Specific epidemiological study designs can be used to reveal etiologic (causal) relationships • First, using observational analytical studies : Determine whether there is an association between a factor or a characteristic and the development of disease • Second: From these associations, derive appropriate inferences regarding a possible causal relationship Analytical Studies • Control and experimental groups • Randomized groups data collected without bias • Dependent and independent factors . Think of going from cause to effect.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). . • 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. .When is a cohort study warranted? • When good evidence suggests an association of a disease with a certain exposure or exposures. • “Generation effect” .Cohort Effect • Changes and variation in the disease or health status of a study population as the study group moves through time. Types of Cohort Studies • Prospective (concurrent) • Retrospective (historical) • Restricted (restricted exposures) . limited exposure.Types of Cohort Studies • Prospective – cohort characterized by determination of exposure levels (exposed vs. • Restricted . narrow behavior (military. long shore men) . not exposed) at baseline (present) and followed for occurrence of disease in future Groups move through time as they age • Retrospective .makes use of historical data to determine exposure level at some baseline in the past and then determine subsequent disease status in the present. Prospective Studies • Also called longitudinal concurrent incidence studies • Looking into the future • Example: Framingham Study of coronary heart disease (CHD) . A study population can be chosen by selecting groups for inclusion in the study on the basis of whether or not they were exposed .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). g. we are comparing exposed and nonexposed persons. where they live) and take histories (e.Selection of Cohort Groups • There are two basic ways to generate cohort groups. blood tests) on the entire population to separate into exposed and non-exposed groups. Select a cohort on the basis of some factor (e.g. .. Select a cohort (defined population) BEFORE any of its members become exposed or before the exposures are identified.. • Regardless of which selection approach is used. Design of a Cohort Experiment . 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. . Framingham Study • Designed to study the effect of multiple factors on coronary heart disease (CHD): age hypertension elevated blood cholesterol tobacco smoking increased physical activity increase in body weight diabetes mellitus . Framingham Study Design • Framingham.500 • Enrollment questionnaire form targeted age range 30-59 years • No clinical evidence of atherosclerotic cardiovascular disease • Cohort re-examined every two years • Problems: white.000 • Study design called for a random sample of 6. middle class . Massachusetts population was 28. work. . Do same tests every year for a period of years.habits. etc. they may have prolonged a life. they refer you to a specialist.Hypothetical Cohort Study Approach 1. 3. Examine people over their life times for contraction of diseases. 2. In meantime. lifestyle. ask questions concerning diet. By screening. Interventional aspect: If positive findings occur. Take blood and do lab tests (as many as possible). . . For example. garbage out • Randomize random selection random assignment • Rule out bias For example. reliable surveys • Critical number of subjects the more. the better Garbage in.Sampling • Valid. degree of accuracy with which subjects have been classified with respect to their exposure. individuals who are sick may be more likely to give the kind of responses that they believe the investigator wants to hear . person Drop off questionnaire Mailed to people Telephone interview Newsletter or magazine .Data Gathering • • • • • Person .to . .Potential Biases in Cohort Studies • • • • • Information bias Bias in estimation of the outcome Bias from non-response Bias from losses to follow-up Analytic bias Your assignment: Describe and differentiate between these types of biases. Advantages of Prospective Cohort Studies • • • • • • • Captive groups Large sample sizes Certain diseases or risk factors targeted Can be used to prove cause-effect Assess magnitude of risk Baseline of rates Number and proportion of cases that can be prevented . Advantages of Prospective Studies (cont’d) • Completeness and accuracy • Opportunity to avoid condition being studied • Quality of data is high • Considers seasonal and other variations over a long period • Tracks effects of aging process . Disadvantages of Prospective Cohort Studies • Large study populations required not easy to find subjects • Expensive • Unpredictable variables • Results not extrapolated to general population • Study results are limited • Time consuming/results are delayed • Requires rigid design and conditions . Disadvantages of Prospective Studies (cont’d) • • • • Subjects lost over time (dropouts) Costs are high Logistically demanding Maintaining quality. validity. accuracy and reliability can be a problem . we use case fatality rate to assess survival For chronic diseases. we use cohort life tables .Survivorship Studies • Survivorship is the number of persons out of a study population who would survive until a certain time interval has been reached • Shows the chance that an event (such as death from cancer) will occur in successive intervals of time once a diagnosis has been made • Analysis yields a cumulative probability of surviving the projected time period For infectious diseases. (Cohort) Life Tables • Charts which summarize the patterns of survival and death in study groups of certain types of disease (chronic) • Insurance companies study these charts very closely. . the survival curve goes down and the risk of death goes up.Survival curves and risk of death for males vs. As one reaches the later years of life. Dip at beginning of life is due to infant mortality rate. . females based on life tables in California for 1980. Is the Association Causal? To be continued… .