epidemiologic study designs nancy d. barker, ms. epidemiologic study design the plan of an empirical...
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Epidemiologic Study Designs
Nancy D. Barker, MS
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Epidemiologic Study Design The plan of an empirical investigation to
assess an E – D relationship.
Exposure• Alcohol consumption
• Raw hamburger
• Smoking
Health Outcome• Breast Cancer
• E. Coli
• Lung Cancer
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Type of Epidemiologic Studies
• Experimental: Randomization to exposure
• Observational: No randomization
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Experimental Studies
The investigator through randomization allocates subjects to different categories of exposure.
Randomization: An allocation procedure that assigns subjects into (one of the) exposure groups being compared so that each subject has the same probability of being in one group as in any other.
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Randomization
• Tends to make demographic, behavioral, genetic, and other characteristics of the comparison groups similar except for their exposure status.
• If the study finds any difference in health outcome between the comparison groups, that difference can only be attributable to their difference in exposure status.
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Experimental Studies
Advantage:• Randomization – unmeasured variables evenly
distributed among exposure• Potential for bias is low
Disadvantage:• Ethical concerns• Cost• Length of study• Not good for rare D
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Observational Studies
• Investigator observes the exposure and outcome status of each
• Most Epidemiologic studies are observational
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Observational Studies
• Descriptive Studies
• Analytic Studies
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Observational Studies
Descriptive StudiesTo organize and summarize data according
to time, place, and person. Why?
• Describe natural history of disease• Extent of public health problem• Identify populations at greatest risk• Allocation of health care resources• Suggest hypothesis about causation
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Observational Studies
Analytic StudiesUsed to quantify the association between an
exposure (E) and a health outcome (D), and to test hypotheses about causal relationships.
• Provides a control group (baseline)• Test hypotheses about determinants• Causation
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Causation
In any research field involving the conduct of scientific investigations and the analysis of data derived from such investigations to test etiologic hypotheses, the assessment of causality is a complicated issue.
In particular, the ability to make causal inferences in the health sciences typically depends on synthesizing results from several studies, both epidemiologic and non-epidemiologic (e.g., laboratory or clinical findings).
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Observational Studies
Advantage:• Natural Setting• Addresses ethical concerns
Disadvantage:• No randomization – investigator can only account
for variables measured.
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Common Observational Study Designs
• Cohort Study
• Case-Control Study
• Cross-sectional Study
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Design Options
• Directionality
• Timing
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Directionality
• When the exposure variable is observed relative in time to when the health outcome is observed.
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Forward Directionality
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Forward Directionality
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Backwards Directionality
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Backwards Directionality
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Non-Directionality
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Timing
• Timing concerns whether the health outcome of interest, and therefore all study events, has already occurred before the study actually began.
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Retrospective
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Prospective
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Retrospective or Prospective
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Cohort Study
• Directionality: Always forward
• Timing: Prospective or Retrospective
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Cohort Study
Synonyms:
• Follow-up study
• Longitudinal study
• Incidence study
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Cohort Study Example
Example:
Prospective:
Framingham Heart Study (ActivEpi)
Retrospective:
VDTs and spontaneous abortions (ActivEpi)
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The Exposure Variable in Cohort Studies
• If E is common, sample from large population, then divide into E and not E.
• If E is rare, sample E from special population, then sample not E from external comparison group.
• Caution: Healthy worker effect.
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Cohort Study
• Advantages– Forward directionality
– Free of certain selection biases
– Prospective cohort less prone to obtaining incorrect information
– Can study several diseases
– Useful for examining rare exposures
– Retrospective cohort study is low-cost and quick, e.g., occupational studies
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Cohort Study
• Disadvantages– Prospective cohort study is costly and time
consuming
– Loss of subjects from migration, lack of participation, withdrawal and death leads to bias
– Statistically and practically inefficient for rare diseases with long latency
– The exposed may be followed more closely than the unexposed
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Example
Measure of Effect• Risk Ratio (Cumulative incidence – risk)• Rate Ratio (Incidence density – rate)
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Case-Control Study
• Directionality: Always backwards• Timing: Always Retrospective
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Case-Control Study
Examples:
• Case-control studies of aspirin and Reye's syndrome (ActivEpi)
• Case-control study of animal food products and Creutzfeldt-Jakob disease (ActivEpi)
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Case-Control Study
Advantages:• Cheaper, quicker
• Sufficient number of cases
• Smaller sample size
• A variety of exposures
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Case-Control Study
Disadvantages:• Do not allow several diseases
• Risk of disease cannot be estimated directly
• Selection bias
• Information bias
• Not for rare exposures
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Case-Control Study
Selection of Controls• Representative of the source population from which
the cases derived
• Population-based controls -Cases and controls come from the same source population
• Hospital-based controls-Easily accessible, tend to be cooperative, and are inexpensive
-Not usually representative of the source population and may represent an illness caused by the exposure
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Case-Control Study
Measure of Effect• Odds Ratio
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Cross-Sectional Study
• Directionality: Always Non-directional• Timing: Always Retrospective
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Cross-Sectional Study
Advantages:• Convenient and inexpensive
• Can consider several exposures and several diseases
• Can generate hypotheses
• Usually represents the general population
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Cross-Sectional Study
Disadvantages:• Cannot establish whether the exposure preceded
disease or disease influenced exposure
• Can identify only prevalent cases rather than incident cases
• Possible bias since only survivors are available for study
• May under-represent diseases with short duration
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Cross-Sectional Study
Example:
Cross-Sectional Study of Smoking and Peripheral Vascular Disease (ActivEpi)
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Cross-Sectional Study
Measure of Effect• Prevalence ratio (PR)• Prevalence odds ratio (POR)
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Hybrid Designs
Combine the elements of at least two basic designs, or extend the strategy of one basic design through repetition.
• The Case-Cohort design• The Nested Case-Control Study
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Incomplete Designs
Studies in which information is missing on one or more relevant factors.
• Ecologic Studies• Proportional Studies
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References
• ActivEpi Companion Textbook; Kleinbaum, Sullivan, Barker
• Principles of Epidemiology; CDC Self-Study Course
• Epidemiology in Medicine; Hennekens, Buring
• Statistics in Public Health: Quantitative Approaches to Public Health Problems; Stroup, Teutsch
• Case-Control Studies; Schlesselman