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Ch. 9 Cohort Studies

Terms in this set (18)

Cohort Studies
The main feature of a cohort study is the observation of a large number of individuals over a long enough period of time to accrue a sufficient number of incident events to compare the risk of developing a disease in the exposed to that in the non-exposed. ex: framingham heart study
OBJECTIVE: evaluate the association between a risk factor and an incident outcome-specifically, compare incidence rates in exposed and nonexposed groups
Selection methods of study populations for cohort:
1) exposed and non exposed- this is the traditional approach for a cohort study and is especially good when an exposure is rare
2) defined population-selection of a population based on some factor ( such as community of residence). advantage of this approach is that we can study multiple exposures and outcomes-but waiting for outcome to develop may require much longer time periods
**either approach is fundamentally the same: -comparing nonexposed vs. exposed - in either case we exclude persons with the outcome of interest at baseline (prevalent cases)
Prospective vs. retrospective cohort design
prospective: exposure and nonexposure are ascertained as they occur during the study; the groups are then followed up for several years into the future and incidence is measured
-retrospective: exposure is ascertained from the past records and outcome is ascertained at the time the study began
Combination of Retrospective and Prospective Design Cohort Studies
Exposure ascertained from objective records in the past
-follow-up and measurement of outcome continues into the future
Framingham heart study
1948-2008, 5,000 from "defined population" study cardiovascular diseases--studied multiple exposures of interest such as hypertension, diabetes, smoking, cholestrol levels- multivariate
cohort: relative risk
RR= {incidence in exposed: [a/(a+b)]} / {incidence in nonexposed: [c/(c+d)]}
Interpreting Relative Risk
RR=1: risk in exposed equal to risk in nonexposed (no association)
RR<1: Risk in exposed is less than risk in nonexposed (negative association; possibly protective)
RR>1: Risk in exposed is greater than risk in nonexposed( positive association; possibly harmful)
Issues with Cohort Studies: Identification/Selection of study population
-information on exposure status and disease status
-comparable/similar in the exposed and nonexposed
feasibility of follow up)-
Issues with Cohort Studies: Prospective Cohort recruitment
-cross sectional survey as initial phase, with cohort formed by follow up surveys
-explicit solicitation of persons with a specific exposure
-recruitment of members of organized groups of persons with and without exposure
Issues with Cohort Studies: Retrospective Cohort Recruitment
-industrial settings with exposure of interest
-other settings with groups of individuals that have extended periods of follow up
Issues with Cohort Studies: Quality of Information of Exposure and Disease
-accurate and complete exposure information
-consistency in assessment of outcome for both exposed and non-exposed
-reassessment of exposure in follow up
-maintaining high level of follow up
Issues with Cohort Studies: Actual number of events expected and observed
-what is anticipated incidence in exposed and unexposed
-sample size for study
-completeness of disease ascertainment
cohort: analysis and interpretation
-computing incidence ( cumulative incidence and/or incidence density)
-computing relative risk (rr), rate ratio, absolute risk difference
-assessing threats to validity : bias and confounding
Biases in Cohort Studies
-Bias in assessment of the outcome
-Information bias
-Biases from nonresponse and losses to follow up
-Analytical Bias
Disadvantages in Cohort Studies
-Not good for diseases of low incidence
-Potential for follow-up bias
-Time Consuming
-Expensive
Advantages in Cohort Studies
-rare exposures can be examined
-wide array of potential outcomes
-temporal relationship clear
-allows for direct determination of absolute risk of disease
When is a cohort study appropriate?
-Available evidence suggesting an association
-rare disease who incidence is high in the exposed
-short period between exposure and disease development
-excellent follow up
-resources available
-randomization not feasible/appropriate
etiology
the cause, set of causes, or manner of causation of a disease or condition.
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