Transcript Epidemiologic Study Designs

Epidemiologic Study Designs
Nancy D. Barker, MS
Epidemiologic Study Design
The plan of an empirical investigation to
assess an E – D relationship.
Exposure
• Alcohol consumption
Health Outcome
• Breast Cancer
• Raw hamburger
• E. Coli
• Smoking
• Lung Cancer
Type of Epidemiologic Studies
• Experimental: Randomization to exposure
• Observational: No randomization
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.
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.
Experimental Studies
Advantage:
• Randomization – unmeasured variables evenly
distributed among exposure
• Potential for bias is low
Disadvantage:
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Ethical concerns
Cost
Length of study
Not good for rare D
Observational Studies
• Investigator observes the exposure and
outcome status of each
• Most Epidemiologic studies are observational
Observational Studies
• Descriptive Studies
• Analytic Studies
Observational Studies
Descriptive Studies
To organize and summarize data according
to time, place, and person. Why?
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Describe natural history of disease
Extent of public health problem
Identify populations at greatest risk
Allocation of health care resources
Suggest hypothesis about causation
Observational Studies
Analytic Studies
Used 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
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).
Observational Studies
Advantage:
• Natural Setting
• Addresses ethical concerns
Disadvantage:
• No randomization – investigator can only account
for variables measured.
Common Observational Study
Designs
• Cohort Study
• Case-Control Study
• Cross-sectional Study
Design Options
• Directionality
• Timing
Directionality
• When the exposure variable is observed relative
in time to when the health outcome is observed.
Forward Directionality
Forward Directionality
Backwards Directionality
Backwards Directionality
Non-Directionality
Timing
• Timing concerns whether the health outcome of
interest, and therefore all study events, has
already occurred before the study actually
began.
Retrospective
Prospective
Retrospective or Prospective
Cohort Study
• Directionality: Always forward
• Timing: Prospective or Retrospective
Cohort Study
Synonyms:
• Follow-up study
• Longitudinal study
• Incidence study
Cohort Study Example
Example:
Prospective:
Framingham Heart Study (ActivEpi)
Retrospective:
VDTs and spontaneous abortions
(ActivEpi)
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.
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
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
Example
Measure of Effect
• Risk Ratio (Cumulative incidence – risk)
• Rate Ratio (Incidence density – rate)
Case-Control Study
• Directionality: Always backwards
• Timing: Always Retrospective
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)
Case-Control Study
Advantages:
• Cheaper, quicker
• Sufficient number of cases
• Smaller sample size
• A variety of exposures
Case-Control Study
Disadvantages:
• Do not allow several diseases
• Risk of disease cannot be estimated directly
• Selection bias
• Information bias
• Not for rare exposures
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
Case-Control Study
Measure of Effect
• Odds Ratio
Cross-Sectional Study
• Directionality: Always Non-directional
• Timing: Always Retrospective
Cross-Sectional Study
Advantages:
• Convenient and inexpensive
• Can consider several exposures and several
diseases
• Can generate hypotheses
• Usually represents the general population
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
Cross-Sectional Study
Example:
Cross-Sectional Study of Smoking and
Peripheral Vascular Disease (ActivEpi)
Cross-Sectional Study
Measure of Effect
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Prevalence ratio (PR)
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Prevalence odds ratio (POR)
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
Incomplete Designs
Studies in which information is missing on
one or more relevant factors.
• Ecologic Studies
• Proportional Studies
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